Both very nice! Depends on you - the Thumb is a bit of a slap 'n pop machine and the back contour makes it more comfortable to play high. The Yamaha is more of an all-rounder. Both excellent basses though.

Ideally you should try both basses since they are two totally different basses - different woods, etc. Price wise the Warwick Thumb and Yamaha top of the range TRB6 (also John Patitucci version) seems to sell in the same price bracket.

I have three 6-strings; two Yamaha TRB1006s and a Cort NA6 (about 10 years old). The Yamahas have 35" scale necks and since I prefer medium-light stainless steel strings; I seem to battle to get the right stuff - something to consider. The Yamaha's neck has a very flat radius and takes a while to get used to as well as the 35" necks, but once you got over that, it is a brilliant bass. I had a slight problem with minor fret buzzes but fixed that up myself - the two trussrod adjustments versus action setting works hand-in-hand and can be annoying if not set right. Took me a week to get either bass set to my liking. I prefer a low action so this makes matters worse! I can get from a Fender Precision/Jazz sound to anything else. The onboard eq is very effective if used correctly - I tend to use the back pick-up mostly and can get close to a Jaco type of sound. For slapping it is awesome and can sound like anything heard on the market. The TRB1006 is a very loud bass as well - the pickups are pretty powerful. PS I do from Jazz to pop and rock. Amp wise I use Eden, Genz-Benz, Hartke and Ashdown products down to a Beringher combo and the Yamahas always sounds good.

The Warwicks I tried were very expensive and probably constructed better than the Yamaha in a way, but in the end it is what feels comfortable to the body and hands, as well as what sounds best. You have a sound in your head you would like to hear and that's where I normally head to. Also, what's the point of buying a very expensive instrument just to look good on stage or because some other well known dude uses that brand - go with your instincts, but do try them both out if possible.

Good luck!

PS If I has a choice, I would have both!