Well, if you get a Rocket Booster, you're almost sure to get one of every card, with the others, a little more risky and probably Impossible with Base. Where as if you go Jungle or Fossil, there's so few cards its almost not worth it because by booster 16 you'll be pulling the exact same cards. But with Gym, I think there's enough cards in it to consider one. I would choose a Gym Heroes or Challenge box, but thats because I have almost the entire set of Base, Jungle, almost all Commons of Fossil and Rocket.
But a 1st Edition Base set would be worth more in the long run, and you have a chance at that elusive Charizard...so I say Gym, but its your call.