Depends on you and how the gun fits... the full sized USP .45 fits me like a custom made glove.
Which is why I got it so reasonably, the person I bought it from (10 year ago) had small hands....
That's pretty much your answer. They're both good guns, albeit a little stressful on some wallets.
I've shot both and own neither. My preference in a similar gun is the XDm. But the P30 would be an excellent alternative
for me (I have medium sized hands, prefer a slimmer frame when all else is equal) and I like just about everything about the gun apart from the price and expensive mags. I'd also add that the M&P and FNP/FNX lines of guns would be excellent options...again,
for me. Everything about these guns are excellent for my tastes and fit in a high-cap service type pistol.
The USP can feel oversized and blocky for a lot of shooters, but comfy if you require a larger grip. Getting each in hand should really answer your question in mere seconds. Whichever feels more natural, wins. It really is that simple.
Not sure if I'll ever own one, but I do like the P30.