The only real downside I see is the cost of ammo for the .357 sig, it's certainly not cheap. If you reload then it definitely makes it more affordable and practical to practice with. I have actually been thinking about picking up a G32 for my G23 and using that for my carry gun as I think .357...
I have just over $800 in mine. It's pretty much all PSA parts with a Primary Arms micro red dot. My first AR and I love it so far, surely only the beginning though!