IT very much depends on what your family's preferences are. Both are fantastic parks and offer a great day out for all ages although each may slightly appeal more to a different age group.
IMHO BG has the better rides, it has three GREAT coaster (plus a couple of "ordinary ones") and some good wate/logflume rides as well as some great animal attractions.
SW has the better shows by far and it's other animal exibits certainly have the edge over BG, but it does basically only have the two thrill rides. The kids seem to adore the chance to hand feed dolphins, sealions and stingrays as well as some other hands in attractions.
Both parks have great kids areas, I think BG gets the edge on the variety, but SW's 3 story high "climbing frame" cargo net is always popular. Basically with under 9s I'd probably go for SW, with teens that like thrill rides I'd go for BG, but the thought of a 90 minute drive there and 90 minutes back to BG might be the clinching though on the debate. If you've had a busy schedule you may prefer the easier drive to SW.