I think compromise is in order. My suggestion (take with a grain of salt) is that you use 4E. Not gamma; just plain ol 4E with maybe essentials even. It's a system you like, know, and can tailor. The players will need to learn it a bit, but with some assurance for a long running game they shouldnt mind the effort. Now for your part, you really need to stick with a long running game/campaign. If adventure path looses its luster, than go west marches sandbox. Whatever you need to do to make the game last.