I love both brands. I prefer HK's but the Sig's aren't bad either. There are some that seem to feel the current production Sig's aren't on par with the older German manufactured ones.
I don't agree as all my Sig's both new and old have all served me well. I don't care for the rails on the Sig's and I really don't like where their marketing has gone but I still love their guns.
I own several HK's including the P30 and USP series. I love them both but the P30 fits me better than the USP. The P30 make the USP feel like a brick!
Both guns have been 100% flawless. If I had to choose between them it would be difficult but I'd take the USP just for the fact it's been around longer and is proven in the field. Not that the P30 hasn't been documented going 90,000+ rounds, but it doesn't have the real world track record yet that the USP series has. This is a personal preference of mine and nothing more. YMMV
They are both outstanding pistols and I wouldn't hesitate to purchase or carry either one.
I'd go with the HK over the Sig IMHO. The Sig will have the better trigger but the HK takes the cake in the long term durability and maintenance department, not to mention the weight difference.