It really is a good idea to go with stainless. The "ceramic" ones are just referring to a coating which is applied over mild steel. The coating is to make the cheap material a bit less crappy.

Also, to get the full benefit of a 2.5" header and cat, the rest of the exhaust aft of the cat should be 2.5" as well. That means custom piping and a muffler with a 2.5" inlet. A 2.5" header and cat bolted up to a smaller-diameter midpipe and muffler will still produce some gains over a stock-sized header, but you're not using their full potential.