I'm a fan of both the Sig Sauer and the Heckler & Koch line. I learned to shoot with a Sig P226 so I've always had a place in my heart for the P series, and greatly admire the older Sig Sauer quality and reliability we have come to know and love. I do admit I'm not a fan of the newer Sig products and I've read here and there about Sig quality eroding over the years, but I've not bought a brand new Sig to experience it firsthand.
I finally bought my first Heckler & Koch this year - a P2000 - and I immediately fell in love with that gun. It felt good in my hand, was easy to shoot, and knocked the center of the target out once I got used to the ergonomics and the trigger/trigger reset. It has, for the most part, been opined that H&K quality has always been high, and I tend to agree. H&K, before their massive introduction into the American market with their USP line back in the 90's, was kind of the revered brand that most of us, including me, couldn't afford. If it wasn't for the influx of P7's into the country, I'd probably never have bought one. The only thing I don't like about my P2000 is the sloppy magazine fit when it's in the mag well. I think it moves too much and I can hear it rattling when I walk. It's enough to unnerve me where I have stopped carrying it for now, but that's probably my OCD side kicking in. As for the H&K 45C, it immediately caught my attention when they were released, and I still intend to get one ... but that magazine issue is something that bugs me. When you pay good money for an H&K product, I expect the magazine not to rattle when it's in the gun.
When it comes down to it, you really can't lose either way. It comes down to what you want and what you expect out of your pistol. Both my P220 and P2000 have been utterly reliable, and I trust both to perform in my most dire time. The Sig has a nice DA/SA trigger that doesn't feel mushy to me, but I also have the LEM trigger in my P2000 so I can't say much about the DA/SA variant H&K offers. The finish on the 45C will probably be more durable than that of the Sig, and you can change the backstraps to fit your hand accordingly with the 45C. I'm not a fan of the short grip the P220 Compact. Just doesn't feel right to me. I also think of the 45C as the 'do all' gun - especially in the woods or in inclement weather. Of course, I think I could take my P220 out in the woods or on a rough and tumble camping trip and it'd do fine, but the ruggedness and 'to hell and back' reliability of the 45C makes me believe it'd be the better performer in the muck and mire.
I favor both the Sig and Heckler & Koch, but if I were in your shoes, I think I'd have to give the nod to the H&K 45C.