My opinion is that Maui is underrated and Cozumel is slightly overrated. I think they're much closer than some people make them out to be.
The complaints most often heard about Maui is that it's expensive and it's less colorful. Expense is up to you and your planning abilities. For example, stay at a condo rather than a resort. As for wildlife, if you love sponges then yeah, Maui doesn't have any, but it has a lot of endemic species of fish and I found the quantities of fish/turtle life to be similar, not the "desert" that some folks reported about Maui.
Coz has a lot of sponges. Sponges are colorful. They're also boring, IMO. I want to see corals and the invertebrates that live in them. I didn't get a chance to see stuff really close up in Coz because the current is swift. It's all drift diving and if you notice something interesting, you get all of about 10 seconds to enjoy it before being swept away.
The water temps and visibility are similar in both places, both warm and pristine, but my experience is that Maui had the edge in visibility, especially in Lanai. You can see the 60-70 foot bottom from the boat and there's little to no current. Amazing.
Topside, it's what you make of it. Can't knock Coz, but also can't deny that Maui is still the US of A. There are certain comforts and conveniences that you just won't find elsewhere.