Well stainless eventually looks almost like rusty alum after time.... I havent personally seen stainless first hand but it seems to be a plus. I guess you could paint it when it does start to turn...

 

I live in Minnesota and have an MBRP 4 inch aluminized exhaust. I have had zero rust on it

 

I have 3" (really just a muffler and resonator delete) I like the way it sounds but will in the future do a 4" turbo back kit with no muffler. But you need money for that and I don't have any of that. I have not heard a 4" system but would assume it would sound deeper than the stock 3" but you will have much better flow with the 4" system as said above. As far as the stainless or alum, I personally go with stainless and the majority of guys I know have run stainless.

Good luck with you decision.