Win/Win
I almost bought a GP100 in 22lr. It had a nice trigger. I came back the next day to trade for it, and it was gone. If the GP100 sights are considerably larger than Single Six sights (which I suspect they are), it will be easier to shoot. At least that's the case with my aging eyes and my K22 vs. Single Six.
My FiL has a very nice GP100 in 357 magnum. He shoots it better than any of his auto pistols.
My 357 Blackhawk is a lot of fun and very accurate. SA revolvers point more naturally for me and I generally like their triggers a lot. The 9mm cylinder is a nice bonus.
A Single Six was my first firearm. This is its replacement, a "rescue gun" that had been neglected/abused. It's accurate and fun to shoot.
I would take any two of the above and be happy with them.
For target shooting, either 22 would be fine. However, for anything involving walking and carrying stuff, I would lean towards the Single Six. GP100's are heavy revolvers. The Single Six is smaller than the Blackhawk, and the Single Six also has an aluminum frame. It's a light, handy revolver. Lugging a GP100 around just to plink with seems like a lot of effort.
Further note: It's pointless to ask how durable any of those four revolvers are, unless the OP is relatively young. If I'd been given those four at age 21 and shot them exclusively, I doubt I'd have worn any of them out by now (35 years later). Ruger makes some very sturdy revolvers. I prefer to buy them used for that reason.
Win/Win