I think the UX50's keyboard is probably the best released on a small Palm/PPC PDA. The HP thumb board is too small in the same way as the TG50's is too small. The UX50 keyboard makes typing pretty feasible in comparison.
Plus, the overall length of the device would make for an iritating hold when using said thumb board. Because of it's length, there is noticably more waver in the top of it when typing (top heavy). With a TG 50, because of it's layout, it wasn't so bad, as the fingers could rest more or less under the screen itself. Who knows, maybe that's one of the reasons Sony decided to stick with a square display on the TG?
With the UX (or even the placement of the NX series keyboards), there was a natural bend located where the fingers could also easily sit behind the screen, to stabilize the device in one's hand.
That's one of the reasons I've never liked a standard PDA with a thumb board slapped on the bottom. It just felt weird using such keyboard extenders. The UX (and the Zaurus 7xx series) is a great form factor for keyboard use if pocketability is important. I imagine there'll be more PDAs utilizing this form factor as time marches on.
Of course, all this is MHO.
As for the other specs on the HP, they ARE good (WiFi, BT, CPU, mem etc) but I'd prefer the UX's smaller screen with higher resolution as opposed to a larger screen with lower resolution. I guess that really depends on one's eyesight, and perception. Other than the VGA display on the Zaurus, I haven't seen anything that looks nearly as crisp as the UX.
Then again, I'd rather own a 27" picture tube TV than any kind of 42" plasma/LCD/projection/whatever as the colour and brightness fidelity really sucks in comparison to a real picture tube. Not worth the sacrifice until HD is universal at least.
All this is just MHO, of course.