You can see the difference in the units in the ph0to gallery by resort area here
http://www.mayanresorts.com/ Don't worry about things like Mayan Island (ownership condos), IQ hotel (not timeshare and not sure if they'rev actually built any). If you are trying to figure out the real difference think "class system".
The way the marketing scheme goes is, the Grand Mayan units and their amenites (including pools and common areas) are more upscale than the Mayan Palace units and their amenities (including pools and common areas) which are more upscale then the Sea Garden units and amenities. Grand Mayan owners are the (present) top of the heap in that Grand Mayan owners can trade/use into Grand Mayan units and facilties, Mayan Palace units and facilities and Sea Gardens unit and facilities. Mayan Palace owners can trade/use into Mayan Palace units/facilities and Sea Garden units and facilities, but NOT trade up to Grand Mayan units or use Grand Mayan facilities at the resorts and the poor old Sea Garden owners get to trade/use just Sea Garden units facilities- unless there is some extraordinary deal on....and they seem to be systematically reducing the number of Sea Garden resorts in the chain I suppose in an effort to up-sell to the Mayan Palace and Grand Mayan units...gotta keep that cash rolling in....and their marketing scheme is always directed to the next up-sell
As for the hard sell that owners are subjected to from time to time (also known as "owner updates" ) they try to sell Grand Mayan owners more Grand Mayan weeks, Mayan Palace owners will be upsold to Grand Mayan or failing that Mayan "Regency" ownership and Sea Garden owners will be shamed (upsold) to Mayan Palace or maybe even Grand Mayan if possible.- because how can you NOT give your kids this opportunity- guilt guilt guilt....
As far as the in-unit specifics, the Grand Mayans have a minimalistic decor, nicer shampoes etc, the cold plunge pool on the balcony and a kitchen that had a 3/4 fridge and 4 burner cooktop, but no oven and a good amount of granite counter space (useable), Mayan Palace units have an undercounter bar fridge, a 2 burner stove and not a lot of counter space (useable only to a point) and no useable balcony (but that can vary by resort and building) and I'm not sure about the Sea Garden kitchen as we've not stayed in the units. And they check to make sure that the classes (oops, I really mean resort guests) don't up-mix (down mixing is okay assuming you're a GM owner or exchnger) by colour of braclet on your wrist. All resorts have their own colour and style of bracelet and owners have a different colour than exchangers..
Having said all this, the resorts, regardless of where you stay, are really nice and well cared for. We bought a Mayan Palace unit on Ebay and we like the decor just fine and our children look back on the one Christmas we spent as exchngers at the Riviera Maya mayan Palalce as one of the best family holiday's they've ever had.
Oh yes and if you exchange through RCI into one of the family of Grupo Mayan resorts and are not an owner, there is a 1 in 5 rule for all Grand Mayan, Mayan Palaces, and Sea Garden resorts in the chain. If you trade in through SFX there is no restriction like that...yet.. But as I said before, great resorts - we like them, but they have somewhat capricious rules and really hard ball marketing to try and sell you.
Just my $.02 worth