Given the choices listed, I'd pick the GP-100 in four-inch trim, in a holster, not a saddlebag. I like Blackhawks but think the GP-100 would be a better all-around choice, and as was mentioned, you can shoot everything from powder-puff 38's to full-house 357 ammo with 180-grain Dino-stomper loads. I don't think weight is as much a consideration for a pack/belt gun (especially while on horseback) as some make it, as long as you have a good belt and holster. Holster needs a thumbbreak or a safety strap, BTW.
Just curious..........is your horse accustomed to gunfire? If somebody touched off a 357 a couple feet from MY ears, I'd be very inclined to buck him into the nearest cactus, or rub him off on the nearest tree RIGHT NOW. Then dance on his crotch with shod feet. But that's just me.
Papajohn