Not to knock AC because so many people here seem to love it, but I suppose it depends upon what kid of traveler you are.
Personally I am more of an adventurous traveler and like to move around a lot so the mainland appealed to me MUCH MUCH MORE.
I have also been to a few island vacations and given the choice between a vacation at AC only or someplace else like cozumel, I'd choose cozumel because the resorts are much better.
As for Belize, if you go to the mainland I think you will find a different sort of attitude in both the natives as well as the type of other tourists you see for obvious reasons.
I went to placencia and liked that much more than AC. Way more laid back and less commercialized (probably due to less and different type of tourist who visits). I stayed at the GREEN PARROT RESORT near Maya Beach which is 10 miles north of Placenica but we had a 4x4 rental to get around in.
Reef is further out and not as nice as that in AC, but the price is the same anyways.
Near San Ingacio I stayed in the Mountain Pine Ridge area, which is maybe 10-15 miles off the Western Hwy on a dirt road. I stayed at the FIVE SISTERS Resort which is next to Ford Coppella's resort. Very beautiful grounds and spectacular views. In fact a couple got married at the base of the falls when we were there.
As for Tikal, I took the tour that the resort offered and afterwards wished I had driven over myself since I could have gotten more time at Tikal. The border is a hassle and I hear it takes almost an hour to get your car over with around $50 in fees to various agents, but less expensive and more adventurous than a tour.
The people we met at Five Sisters, Green Parrot, and every place in between were great and we made lots of new fews. By contrast we only met 2 couples worth spending time with at Ramon's.
If you have a car, don't miss the belize zoo, jaguar reserve, blue hole and herman's cave, carocal, rios fios caves, barton creek cave...etc etc etc.
Good luck on your wedding, I think you picked a great place to jump into your honeymoon! In fact we met a lot of adventurous newlyweds down there!