I think you made a good choice. We have been on Mardi Gras twice and Celebration once. Our first cruise on Mardi Gras we were also mid-ship, deck 9. It is a good location (but we have also been in a Havana Cabana and aft, deck 9 on our other Excel cruises, and all were great locations). I agree with you on staying deck 9 (or lower) in regard to the elevators.
I see you are cruising over a holiday, so likely a very full ship (we were over spring break on a completely full ship). In our experience the Excel class doesn't seem as crowded when sailing full (the crowds on the Dream class kill me when sailing full).