Shoot the ones you think you might like the best, and go with the one you do.
I have both the guns you mentioned, and both will serve you well. Both are just slightly smaller than a Colt Commander, and they all carry the same. I doubt you'll notice any difference in that respect. I went from Commanders to a standard P220 when I first switched, and never had any problems.
Do you need the SRT? I dont know, do you? I dont think they are necessary, but some seem to think its important. Again, try and shoot both, and see what you think.
Personally, I'd take the P229 (357SIG) over the P220. The main reason being it carries more ammo in the mag and the grip is slightly more comfortable. Other than that, there is really no difference.