2.25" or 2.5" exhaust?

While 2.25" may be adequate, if the cost is similar go with the 2.5" pipes. Unless they are mandrel bent the pipes will be smashed down to 2.25" at each significant bend anyhow. Larger diameter pipes will tend to be a bit louder with the same muffler, but will have a nicer tone. If you want pipes that "snap" go with a smaller diameter.

If your muffler guy tells you that you should run the smaller pipe because "your engine needs some back pressure" he is lying to you (although he may be too stupid to know it). Take your business elsewhere.