I've given this one a lot of thought. I've been a SIG owner and shooter for close to 25 years, and find that it's almost like picking a favorite child. My gut reaction would be to say the 220, especially the old West German stamped and pinned European model, a gun that I loved and just replaced, but with all things considered, I'd have to give the nod to the P6.
Baiscally the P6 is a cut down P220, so you have the excellence of the basic platform as a starting point. The loss of a two rounds from the 220, as well as going from .45 to 9mm, should give the edge to the P220, but the size reduction more than makes up for this with the added option of being able to carry the P6 much more easily than the 220. Plus, the price of the surplus P6 really makes the difference, an absolutely incredible price for a SIG, putting many who say that they would never pay as much as a SIG costs into the "put up or shut up" category. Also, any "deficiency" in the P6's heavier trigger can be handled with a reduced power Wolff spring, an alteration I haven't found necessary to do yet.
I think you'd also have to strongly consider the P226, but the full size of the gun reduces practicality, again with the nod going to the smaller P6. Of course, that's not to say that I'm giving up my 226 DAK in .357 SIG any time soon.
The P6 feels perfect in my hand, my pinky nestled up agains the floor plate. The gun is a smooth shooter, very accurate, and has been dead reliable. Of course, the downside to the P6 is the issue with hollowpoints, but PowerBalls settle that issue nicely. Also, with the P6 you have a West German stamped and pinned gun, an aspect I find to be most desirable. Why? Maybe it's nostalgia, but I really think its more the balance of the gun. It's hard to put my finger on it.
Finally, these guns have a history. I really enjoy police trade-ins and find value in that they are often lightly shot. Mine is stamped 10/78, and that is a lot of years ago when I was in my second year of college. These guns have been around, and are usally worn enough (but not too much) that you can enjoy them without having to baby them. The only change I have done to mine is to swap out for Hogue hex head grip screws. I would also like to find a Call type white ring front sight as I think it would go perfectly with the plain black square notch sight offering quick acquisition and an easy to read sight picture.
The SIG 239 gets a big honorable mention.
