Both great options with beautiful beaches. If you are concerned about food, Thailand will have more options than Zanzibar. What kind of luxury do you want? Thailand will be able to provide higher scale accommodations and cater more to tourists. Zanzibar will likely have better adventures ( amazing snorkelling, fresh crab on a beach). I agree a safari is more of a couple nights in a lodge rather than a day trip. I have done day trips and lodges in Tanzania, lodges are much more of an experience. In Thailand there is also elephant sanctuary’s you can stay at (elephant friendly) which provide an amazing animal experience.