HK or SIG? If you have the money, both. If not, you have to determine what role the pistol needs to fill. Several of the posts have stated that the SIG is overpriced. I guess that depends on the gunshop you are standing in. In the Mesa, Scottsdale, Pheonix area shops I have been in lately, the HK USP Tactical is the hands down winner for the "Most Outrageously Overpriced Pistol" award. The HK Tactical, with soft case, cleaning kit and one extra magazine was going for 875$. In this same shop a SIG P245, with hard case and one extra magazine, but no cleaning kit, was going for 599$. How HK can get away with selling a cleaning kit for 275$ is a trick I'd like to learn.
One thing that has not been mentioned is Customer Service. SIG has an excellent customer service reputation. HK on the other hand has a reputation for slow service, and rude customer service representitives.
Before you make up your mind one way or the other, I suggest you talk to your local PD, the SWAT guys in particular, about their experiences with the two firms you are looking at. I talked with SWAT Team members from Mesa and Gilbert, and both departments have had similar experiences as related above. Both are great firearms, just know what You want before buying.
Myself, I would buy the HK USP Tactical. Like yourself, I already have several SIGs, and the USP would just help diversify the gunsafe.
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May your lead always hit center mass and your brass always land in your range bag.
~Blades~