The GP100s are good guns but don't overlook the older Security-Six models. The Sec.Sixes are very similar to the GPs and S&W 686 in size and weight and are often very modestly priced. I picked up a 4" stainless Sec.Six, like new condition for about $200, last fall.
I am, above all, a huge fan of the Blackhawk and have more than a couple dozen of them, including many Single-Sixes. The stainless models weigh a few ounces more than the others as they have also a stainless grip frame, where the others, except Super Blackhawks, have alloy grip frames. I prefer the extra weight of the steel grip framed guns over the alloy, while other folks appreciate the weight savings of the alloy gripframe.
The .357 Blackhawk is an unbelievably robust handgun that will eat the heaviest loads without a problem. Because it is the exact size of the larger calibers, .44 and .45, it is also somewhat heavier. The extra weight makes shooting full loads a pleasant experience.
If you are not put off by fixed sights, take a look at the New Vaquero. It is built on a smaller frame than the Blackhawk. It has the look and feel of the old Colt Peacemaker, but with an affordable price tag.
Lots of folks pay a little extra for the anticipated versitality of a convertable, but most never get around to shooting the spare cylinder. While the concept is intriguing, unless the ammo is free, dropping back to a considerably less powerful cartridge that also requires changing your sight settings, becomes too much trouble real fast.
Why have just one? Have one of each