You have two different topics here:
tubular vs clincher
carbon vs alloy

tubular vs clincher

Tub riders will tell you the ride quality is better. The big difference is what happens with a puncture, A tubular retains it's shape and you some control with the ability to ride a little further after a puncture. A clincher, it goes flat, you stop. On Saturday during a race, my rear went, sounded like a gun going off, blew the clincher off the rim, jamming the wheel and I skided to a semi controlled stop on my alloy rim. Only because I was cruising at 25kmh after getting dropped, a couple of laps earlier at 40kmh in a small bunch it would of been a disaster.

A clincher is easier to fix after a puncture, just bung in a new tube, but that assumes you survive the results of the puncture, no drama going uphill but down?

carbon vs alloy
You can get 800g tubular carbon wheelsets now, or cheap chinese carbon clincher 38mm wheelsets that weigh in at 1200gm for around $600.

The advantage of carbon is weight, rotational weight. It is much quicker to accelerate with light wheels.

The advantage of alloy is braking, both in the wet and repeated braking. The horror stories you hear about carbon wheels on long descents is from people constantly on the brakes (ie riding in big groups like Grand Fondos), because carbon does not conduct the heat well, resulting in blown tubes on clinchers, melted glue on tubulars and buckled wheels in extreme cases.

First hilly race on carbon clinchers, 6km downhill to finish, chasing a couple of riders. Almost had a clip stack after the finish, brakes where too effective and my legs shaking too much to unclip.

tubular or clincher
carbon or alloy
it is your choice, it is not carbon tubulars vs alloy clinchers, it is also includes alloy tubulars and carbon clinchers

I not comparing cheap Chinese tubular rims or carbon clinchers, but I am comparing the performance gains of a quality carbon tubular wheel versus mavics high end clincher wheelset when there is only 50g difference in weight.

As a previous post suggested, go search for a spreadsheet or similar that will enable you to calculate the exact amount of time gained by the 50 grams lost.

If we take away the rolling resistance (which will probably also mean almost nothing time wise) and the increased comfort/subtleness/whatever of a tubular i am sure will come to the decision that loosing 50 grams wont matter. But there are other factors much more important when choosing a wheelset...

_________________
"Stay cool and try to survive" A. Klier to the other members of the Garmin classics squad the night before P-R.

I know the time differences will be negligible. That's exactly my point. So what are the other factors, are they merely cosmetic, ie carbon bling or are tubulars worth the time and additional financial cost when a puncture occurs. Or is it some people just stick to a certain brand. I don't expect anyone to have a definitive answer, nor do I, but I do believe with the advent of lighter aluminium clincher wheels, the mid range weighted (1250g) carbon tubular wheels are becoming old. Unless you can buy the big brand wheels at a discount or pay a fraction of the price for gigantex rims laced with whatever hubs you choose there seems no real advantage unless riding tubulars means that much.

Thanks for all the input.

With respect your question is all over the place.

In the OP you state "Purely in the context of climbing, ie sportive or climbing big cols for fun. Not for road racing or aerodynamic properties."

Compare that with the above statement you've just made.

That's before considering the bizarre idea of a sportive with only climbing ... one in which aerodynamic properties are irrelevant. LOL.

So, to clear up your confusion, tubs are lighter, safer, and corner better. Clinchers are generally regarded as more practical and the puncture fixing costs, as you say, are lower.

Mods - please lock thread!

almost 100% of answers you need have been made. If i may add:

I made this purchased decision a few years back.

decided to buy Mavic R-Sys (1390g) came with Ti Mavic QR (108g) for KCNC (45g)

use Bontrager SL inner tubes

GP4000s tyres.

I have been supremely pleased with every aspect of these wheels, an i feel they can not be touched for two reasons:

They have im afraid a bad reputation due to the first version of the spokes being to week. This has really spoilt the name of R-Sys. mine have been fine and are like a rock! this means they are always sold cheaply new, as mine were meaning when your considering wheels, these will be amazing value for money.

Secondly, for sportive rides, ones ive taken part in often have a Mavic tent/ trailer on site, and they are so eager and welcoming to you if you have a small (i needed a cassette tightening) or big problem. This back up, support adds value to me.

Finally i find them very comfy while being siff, how do they do this i dont know!

all the best

Off topic, but what puts me off all of Mavic's wheels is the stupid nylon bushing in the freehub that wears out and causes screeching. I think it's a bizarre design to use on high end wheels.

Older carbon braking surfaces did tend to suck at braking, whether descending or just general braking before a corner, stopping at a red light, etc. Newer carbon braking surfaces can be just about as good as aluminum, IMHO. For instance, I ride Enve Smart System wheels and the braking is great. I am an 80kg rider who tends to make up ground lost to the climbers on descents, so I am talking about speeds of 70-80kph or more with no braking issues. Admittedly, I am in sunny Southern California so haven't had to brake in the rain on these wheels, but I will be running the aluminum training wheels in the rainy season just to keep the Enves from dealing with winter crud.

_________________
R5 VWD | 2008 SRAM Red | Rotor 3D+ SRM | Zipp 101s/Enve 3.4s
S5 Team | 2012 SRAM Red | Rotor 3D+ SRM | Enve 6.7s
P3C | Zipp Vuka Aero/R2C | Omega | FSA SRM | Zipp 808/Super 9 Disc
T1 | Scatto | Rolf Prima FX58

and I answered in a very generic way given the broad assumptions you made

Lets try and answer the simple question is the Mavic R SYS CL clincher a better wheelset for you than a Vision TriMax Carbon TC24 tubular

Tyre choice, your choice of which clincher + which tube or which tubular tyre is going to affect overall wheel weight, either amplifying or negating the difference between wheels. As well as making a bigger impact on ride quality that wheel choice.

Assuming tyres do not impact on the weight. A better physicist than me will tell you 50gm saving in weight will save you X watts climbing 1km at 10% under laboratory conditions which translated to you arriving at the top of Alp D'Huez Y seconds faster with Vision TriMax Carbon TC24 wheels over the Mavic R SYS CL. I would say X and Y are small numbers.

Punctures, going uphill I rather be on clinchers, easier to repair, going downhill I rather be on tubulars more control of bike with a flat.

Braking, do you find you are constantly on the brakes going downhill, because
a: you are a nervous descender
b that is the way you ride and the roads you ride
c in sportives the others are poor descenders
d you ride in big bunches

If you answers yes to any of them then get the Mavics. Though if it was A consider a good pair of alloy tubulars

Do you ride like a lunatic in the rain?
Get the Mavics, better braking in rain on alloys I am told.

Convenience the Mavics are easier to change tubes and tires on, the Visions easier to get a replacement spokes for.

There is no reason why you should not get the Mavics, as they do what you want, though if your budget can stretch to some Zipp 303 Firecrest Tubulars, just as fast uphill as the Mavics and much faster down the other side