I've only been to Plaza Gardens and Agrabah Cafe, both were lovely. Agrabah certainly has more unusual food and the atmosphere is great. There's a great selection of starters including couscous and various salads. I can't really remember the main courses, since I ate so much from the starter buffet.

But I recall some lovely fish and you could make your own kebap sandwich. Dessert choices were varied and included a delicious pecan pie. The restaurant was very quiet every time we visited - no crowds at all. We didn't even have a reservation. The only drawback was the staff - both times I found them ..well, kinda unmotivated. They weren't unfriendly or anything, but just lacking something - with the exception of the chef, he was funny and great, chatted with us a bit.
Plaza Gardens on the other hand has more safe choices, but still a great buffet. We too saw snails and the had oysters too, but my friend said they weren't that good unfortunately. But everything else is really lovely. You got salads, fish, various meats, pasta, pizza, a bunch of side dishes, soup, freshly carved meat and a wide range of desserts - I'm especially fond of the chocolate mousse. The theme of the restaurant is really great and although it was packed everytime we went, it was never really loud or hectic. We went two times this year, Easter holiday, had no reservation. They got two queues, one for people with reservations, one for those without. You have to wait a bit without reservation, but it's no biggie - waited for 10 minutes, at most. Your experience may of course vary.
And the staff are absolutely lovely - friendly, smiling and warm.
If I had to find something negative with Plaza Gardens it would be the speed of the service - we waited really long for our drinks, which was kinda understandable given the full restaurant, but a slight drawback nonetheless. We had already started with the main courses when our drinks arrived. Waitress apologized many times and we made no big deal of it.
Oh, and at Plaza Gardens you can choose unlimited drinks too for 2 or 3 Euros more, if I remember correctly. Depending on how thirsty you are, this can really pay off, imho.
So in conclusion I can recommend both of these buffets.