I have both and have been going back and forth about which to use for duty carry.
The grip on the P30 is more ergonomic and the recoil impulse is more traditional and linear with the single spring and plastic buffer. The rail is standard and has enough slots to fit pretty much anything. The magazine release is bigger. The trigger, however, is not modular like the USP/HK45 series and has a longer reset. There is anecdotal evidence that the long slide 9mms are more susceptible to malfunctions from limpwristing and debris due to the extra slide weight, but that generally holds true for all polymer pistols.
The USP can fit most accessories with an adapter, which may limit holster choices. The recoil impulse may be smoother, but some users report a "figure 8" movement of the front sight due to the double recoil spring setup. The trigger and reset are better on the USP (or readily modifiable to be), and jet funnels and their 18rd mags can be used.
For me, for duty use, I'd be comfortable with either, but am probably going to try the USP first and see how it goes. If I don't end up liking it, I'll probably send the P30L to lazy wolf and just deal with the 15rd mags and enjoy the bigger selection of retention holsters.