ypsilon you beat me, but anyway:
over the past few years every rear wheel with a dt 240 or 190 hub sucked in tour tests because they don't like the flange spacing. in the actual test the vision wheels are surprisingly good(they have very narrow flange spacing)but few pages before the test vision has a full page of commercial for these wheels while i can't remember dt adds in a tour mag...
from what i've heard(also from garmin team members who used zipp/powertap wheels in 2009)powertap rear wheels are not the stiffest because you have to use a certain lacing pattern(don't know if it's radial drive side or 1x non drive or whatever)and a wheel expert would probably take another lacing pattern to make a stiff wheelset, but then the power measuring does not work the way powertap wants it).
i've never felt that my wheels with dt hubs were not stiff enough, and i used a powertap rear wheel(ea90 clincher rim)for training in 2010 and that was also ok for what i used it for.
i also don't think a 4 cm rear wheel can be of any problem in wind conditions where you still ride your bike(you don't ride in hurricanes, right?).
training and racing with power will make you faster than whatever stiff or light wheel you could find on the market in the long term, so if you can afford it go with the carbon rim(or wheelset, because that's what you want...). the velocity rim with aerolite or cx ray bladed spokes will also make a cool wheel though but the only thing you will really feel is the lighter weight of your wallet and the nicer look of the bike if you take the carbon route.

Many pro riders run a Zipp 303 front and 404 rear or 404/808 mix. If you are going to run a shallower front, for aesthetics I would recommend running two wheels from the same manufacturer. So either run a Zipp 202 front and 303 rear (or something similar from your favorite wheel company).

_________________
"Let's plunge ourselves into the roar of time, the whirl of accident; may pain and pleasure, success and failure, shift as they will -- it's only action that can make a man."

On the right track but motion has a lot more to do with it. Exerting a side force to a bike and rider feels like you get pushed over at the front because all the mass of bike and rider is following the front. It's an efficient way of manuvering mass, think of the motor car for example....you are steering the mass the way you want to go. Having rear steering would not manuver the mass efficiently enough. And as such you can have a full disc at the rear which causes little effect on manuverability. Pros do not ride disc front unless they are on track in doors where nill or very little wind is to be had to exert a side force.

Again, the guys who do not ride a disc are just having a guess and pocking at ideas. No I am not an aero or physics expert but have ridden a disc wheel enough in varied conditions in TT racing to know what I am on about.

Shleck is a surprise that he stacked it but if you look in the history of pro cycling mountain-goats generally have poor descending skills and are no good at TT bikes. Don't know why they are not as good down hill but on a TT bike they have less weight to hold the bike down and so are not as smooth as the bigger riders. Look at Indurain, Cancellara, Ullrich, Martin....all bigger guys riding smooth TT. Then look at Shleck, Rasmussen and they are gittery.

_________________
It's not how much you spend on a bike it's how hard you can ride it.

Thank you Simon.

Great to hear this from a Pro

_________________
Baum Ristretto, Baum Mondrian Corretto & the '89 Merckx! | "If you could eat it in one sitting, hold your line." | twitter.com/Tinea_Pedis

PowerTap specifies a 3x on the NDS. You can have whatever lacing you want on the DS.

_________________
It's not how much you spend on a bike it's how hard you can ride it.

Yes, thanks! If the powertap rear was "ok" for simon I don't think I'm going to be having any problems...

I'm still thinking about a rear-wheel-only solution because two decent carbon rims is a very expensive option and I might prefer to defer the "big spend" until I get some tubulars. Also I'm pretty sure that for everyday training at least I'd prefer to have an alloy rim on the front for braking.

An idea I've been toying with, which I know some people are going to think is the worst of both worlds but which could also be seen as the best depending... is getting the powertap G3 built up on a Reynolds Thirty Two carbon clincher. I know that there would be few very obvious benefits over an alloy clincher, but (1) it would be significantly lighter for the same rim profile (32mm) than any alloy clincher, offering at least a little aero benefit and possibly some rigidity(?) over a box-section alloy clincher like the velocity A32 (2) it would go pretty well with my black alloy eurus clincher on the front (26mm), replacing the 30mm eurus rear, and I'd still have perfect braking at the front in all conditions, not have to worry about overheating etc (3) because it's only one wheel it wouldn't be too expensive, and I'd have a pretty versatile, high performance all-round wheel.

>If the ENVE rim with unknown spokes and DT hubs didn't test overly stiff, that doesn't have to have too much to do with the rim.

While that is true, I recall Roues Artisanales also tested Enve (wheels or rims I can't remember) and they certainly didn't call them stiff, quite the opposite.
So two tests vs. buyer/user perception.