Like others said, apples and oranges. I've been to Fiji three times and Cozumel twice. In Fiji you will see different and huge variety of fish, corals, critters and a whole world of soft corals. Fiji is more expensive. Fiji is more attuned to metric system and non-American divers although everyone speaks fluent English. So PSI might be BARs. Meters not feet. Not much of an adjustment really.
Nadi is not the place to dive. As other said...go to Taveuni or Bequa ( shark dives too in Bequa) or Namena ( fantastic place) or Bigh waters or Kandavu. The liveaboard Naia gets rave reviews and hits a couple of those. Most places require a long taxi drive from Nadi or a flight on puddle jumper.
Cozumel is usually clearer, easier, more set up for diving and the food is better. It's run for divers from the US. You, after awhile, tend to see the usual carribean creatures there while Fiji probably has 5 times the diversity.
People are very friendly in both places...maybe Fiji gets the edge.
I was in Fiji in April and also spent time in Vanuatu, another awesome destination btw. Did Bequa shark dive...touristy but amazing.
Caveat: Several areas like Taveuni, Raki Raki and Savu Savu were hit hard by Cyclone Winston so do your research and inquire. Diving probably not damaged but your resort might have been.
Bula !