I'm going to get a set of wheels built up. I'm thinking Ambriso F20 tubular rims with 28 CX Ray spokes. I've narrowed my choice of hubs down to PMP, Royce and Chris King. But with nearly £200 diferance between the Chris Kings and the other two are they really worth that much more?

PMP - acceptably light and not seen too often, but seems bit dated in design these days and good luck with customer service.
Royce - very well engineered and roll like hot butter, but known to be portly on the scale. They are a GB brand, if you are feeling patriotic.
Chris King - yes, they are worth the extra money...

Why not go for Novatec or Monkey Circus hubs? Both very nice and very cheap.

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What Stolichnaya said.

Go Chris King.

PMP are very heavy, and a bit temperamental, with rubbish service.

Royce are beautiful and look very traditional, but heavy.

Nothing bad to say about King.

I have had pmp hubs for 5 years. They are not light by todays standards but light enough for simoni to win the giro on in 2001. Service depends where you are. Wheelsmith stock them and sell parts. I got a rear hub spacer from them. Briliant service from Derek got the part in a day. All parts like axle, freehub are all replaceable. They are smooth, much smoother than tune, we compositi, extralite. They will last.

Royce fantastic machining, super smooth, great service. Not keen on the aesthetics.

Chris king, I am sure they are great, but you can get some nice tubs for 200 squid.
What about nos campy?

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Owl

Not what you asked, but, to keep the UK theme going, have you considered Hope? They have good reputation here, and have a fair price/longevity/weight ratio.

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I'm going to have them built by Derek at Wheelsmith, so PMP could be an option, it's now down to them and the Kings.

I had thought about Hope but he doesn't stock them.

This is a bit of a once in a life time dream bike so the Chris Kings are edging ahead.

I would say it depends on the type of bike you're going to be putting these wheels on. If you're going for a lightweight weenie build, the Chris King are THE hubs (of those you mentioned in contention). If you're building a stylie, keep forever, with smoothness and durability in mind, I would go with the Royce hubset.

Royce and PMP are the same weight if I remember correctly. I have been very impressed on how smooth my Royce's are.

King--Should last you 20+ years, long as you o'haul the bearings occasionally.

The wheels will be going on a Wilier Centro Uno with a record group

In what way are they temperamental?

I must say, I haven't rolled with them yet, but just built a wheelset with PMP and they look extremely good, as in super-smooth, well finished, well designed mechanically. 400g for the pair, which is relatively heavy but not stupid and less than e.g. Royce (which comes in at ~470g IIRC). At this stage, I'm very impressed, especially also considering the price.

i'd go for the loudest cassette body i could find, but thats me

PMP hubs are not temperamental in my 5 years experience. There are other owners on this forum who would also say the same. They are very reliable and fairly unique in that it is easy to get replacements parts for most aspects of the hubs. A valid criticism that you could level at them is that they are boring and not as sexy as some of the newer super-light hubs like dash. I have bought some excellent parts in the last 12 years and i have bought some parts I would not buy again. I had a poor experience with WR Compositi hubs that we very bling and sexy, but were awkward from the start and the front hub went on the flange within 3 years.

There is a sense of terroir with combining italian hubs and rims, I always thought Campag Record 9/10sp hubs look great on F20s, many riders in the early 2000s thought so (Popovych in his unusually strong year of 2003 used this combo at the Giro).

Chris King parts are fantastic and superbly made but will you ever have that nagging thought you should have tracked down those NOS Campag Record hubs? FWIW the new Campag Record hubs are incredibly ugly!

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Owl

They are Owl, but they run very nicely; if only there wasn't a 300g weight diff 'tween record on Open pro and my Elites, darn it to murgatroyd.