Light rail on the USP is the goofy universal rail that has to use an adapter to convert it to picatinny, HK45 has the standard rail so your lights will work out of the box. Trigger is much different feeling in the HK45 over the USP and doesn't have the factory match trigger option. The trigger on the HK45, P30, compacts, etc are not horrible when comparing DA/SA to DA/SA USP's but the USP is worlds better. Factory Match trigger is on par with any custom tuned trigger in other guns pretty much out of the box when comparing apple's to apple's, obviously its not going to feel like a 1911 trigger.
On the USP series you have the ability to run many different types of trigger systems, LEM-SA/DA-Even SA/DA no decocker so you can run it condition 1 cocked and locked. The P series and HK45's can run many combo's also but a tad more limited so not enough argument on that.
Mag compatibility with the HK45, HK45C, and USP 45 compact makes sharing a little easier on the wallet. The USP 45 does have the benefit of +2 rounds of ammo even though it comes at the cost of a larger grip area. Im still in shock that HK can make arguably the most comfortable grip in the world on a handgun and F up the trigger. Seriously how can the trigger get worse compared to the USP's???????
Currently own a USP9 and USP Tactical 45 and dad has the HK45CT, we both want the HK45T so answer is always GET BOTH!