It really depends on what your criteria is. If you're trying to be thrifty...Forbidden Island. If you're playing with younger kids...Forbidden Island. If you looking for something easy to setup and put away...Forbidden Island. If you want something a tad more intricate and complex... Pandemic. If you're playing with teenagers and up... Pandemic. I would factor in the Pandemic expansion, "On the Brink" as well. It really makes Pandemic a great game even better! I like both.
This is better than pandemic. Pandemic plays less "cooperative," and is much less forgiving for beginning players. Beginning players will become frustrated with the difficulty of pandemic and how little you seem to do while others who are more skilled with the game domineer and control the gameplay.
Start with Forbidden Island. If you like it and would like to step up, Pandemic is the way to go. Adding the On the Brink expansion to Pandemic brings even more thrills with a hidden terrorist player, who battles against the rest of the players.