2.5 inch or 3 inch

I feel 2.5" would be plenty for what you have and what you are using it for. I don't know what size you have now but if it's 2.5", I wouldn't change it.

If you NEED new exhaust go with whichever you want, but don't expect big gains from the extra money you spend for the 3" (you may see nothing at all from it).

Really….. The extra info you gave is still very vague.