O.K.....I'll part with the majority here.....I have 3 kids, currently DS-10, DD-6, and DS-4, and BB is their hands-down favorite. I'd recommend BB for a 4 and 6 year old who are good swimmers for two reasons (a) Jr. Ski Patrol area, and (b) family raft ride. The Jr. Ski Patrol area is specifically designed for kids who are starting to out grow preschool areas but who are not big enough for the really intense water slides. It has several medium innertube rides, a 'walk across the ice bergs thing', and a T-bar that the kids hold on to and then fall off of. I've read in books that it's designed for kids 8-12, but I think it's really more for the 4-9 year old set. I think 10 and up tend to be on the other rides. The family raft ride at BB is also fantastic.....large round rafts that accomodate a family (all 5 of us fit)....whole family can have fun together....even our pretty unadventurous 3 year old enjoyed it last year. There are several other medium rides that 4-6 year olds can do as well, e.g. Toboggan Racers.
TL does have a more tropical feel....nicer landscaping, more restful...features which are certainly appreciated by parents but not kids.
TL does have the really awesome wave pool though.
Go to both if you can!