Hi Andrea,
If you want to focus your trip on Epcot (which is where Illuminations takes place in the evening, the show I believe you are referring to) and Animal Kingdom, I would purchase a four day Magic Your Way Base Ticket. As long as you stay in one park each day (and don't mind not switching between parks in a single day), this is the most economical option. You can return to your resort mid-day for a swim, to have dinner, rest, etc. and then return to the same park in the evening.
If you are referring to Fantasmic (another evening spectacle) that makes use of lazers, dancing water and fireworks, that takes place at Hollywood Studios. Either way, a Magic Your Way ticket is the least expensive option. If you want to be able to switch between parks intraday, you can purchase the Park Hopper Option.
For more information on ticket options, including current prices, click
HERE.
Have a wonderful trip!
Tanya