Depends on how early you wake up each morning. One Fish Two Fish has about an hour wait at peak times of the day. If you show up early, the kids can ride it first.
You could always visit your first day to get an idea of how things are, then buy express pass booklets if you think you need them.
Besides the FOTL, staying onsite give you the neat pools, plus you "feel" like part of the action. But for $200 a night differance, it's a tuff call.
As the kids get older and make the height requirements for the coasters, you'd definately want to stay onsite and have FOTL. But they'll probably want to spend a lot of time in the play areas getting soaked, so you don't need FOTL for that.