I don't think you can go wrong either way, but one may be better depending on how you plan to use it.
From what I can tell only the .45 is actually the Thunder Ranch gun, while the .357 is a 627/327 Performance Center model, not that that should make any difference (merely product branding).
From my perspective...
Advantages of the .45:
Legal in IDPA (5" barrel of the .357 is too long)
you already shoot .45, so purchasing is easier and reloading is cheaper
I'd prefer .45 for HD since it very potent without the intense recoil/sound/flash of a .357 (V-comp could be seriously loud)
reloading with moon clips can be done very quickly
Advantages of the .357:
2 more rounds
.357 may be easier to reload than .45 (I don't know enough to say though)
longer barrel should be more accurate
.357 can be better pressed into hunting and longer range target shooting (if you are into that)
Keep in mind I have not handled either of these guns, but I sure wouldn't mind either of them!