Okay, I've done both. And I will list what I think are the main differences with both types of vacations.
1) You can get some really really good all inclusive deals, of course you can with a cruise too.
2) The main difference I think would be that once you pay for your land trip, that's it, you don't really have to spend another cent. Literally. From the time you get on the plane until you get back on the plane, if you don't leave your resort, you don't have to spend a dime. No tips, no paying for drinks and no little extras. A good all inclusive is just that, ALL INCLUSIVE. But expensive too.
3) All drinks are included on the all inclusive and not on a cruise. Now when we went on our last all inclusive, this was a big deal. We were younger (somewhere in our late 20's) and liked to drink and party, so knowing it was all included was awesome, we even drank just because we could, silly I know, but we were young. On our Carnival Miracle cruise in April, we each had 2 alcoholic drinks and not because we had to pay for them, because that isn't our focus anymore (having 5 kids in 8 years will do that to you. lol) We just wanted some R & R, good food, entertainment and warm weather. We got them all and then some on a cruise.
I think the biggest difference is buying the drinks on a cruise and not having to pay tips, but I think the overall atmosphere, food and entertainment on a cruise far outweighs an all inclusive. Especially the shows and games on a cruise. The live shows at all inclusive's are not any thing special. I've seen some fantastic cruise performances that were a great part of our vacation.
If you're young, want to drink and party and sit on a beach, go for the all inclusive, if you want your vacation to be a whole experience with food, entertainment, new ports to explore and many different excursions a cruise is for you.
Comparing both, I'm a cruiser for good now. I can go on a 7 day cruise and see 5 or 6 different countries, eat wonderful food, take in fabulous shows, sit by the pool with a band playing, go to the casino, lay on a beach, swim with dolphins, snorkel with sting rays and enjoy some of the worlds best scenery from 12 stories above the ocean on a floating resort. In my opinion there is no choice. Yes, resorts offer most of this too but not with that, all in one place, floating along the ocean feeling you get with a cruise.
So... I'll forgo all those free watered down all inclusive drinks and gladly pay for the strong cruise drinks, in the end the price is about the same and I enjoy the cruise more.
Tara