This will get a million hits on the Cozumel section of this board, but has been answered as many times there so you can probably check it out without posting even. They would ask what type of service do you want (more specifically) and then it becomes a matter of opionion. Rest assured there are many and most are great. If you really need more I can offer my opinion but it is just one of many.
Regarding nitrox, in 5 trips we've never used it and I don't know if I've ever seen anyone who did. However, again it would depend on how much diving you plan to do and depths/times. For most folks the normal two morning boat dives each day is the routine. In that case it probably is not worth it, but you will have to do some calculations based on your plans.
You will love it either way, and FA is very nice.
Bob