I'd say Islands has the better coasters, but remember there are only 3! If you are looking for volume, go for BGT - and I'm certainly not saying the coasters are poor there at all, no way! Montu is the best inverted I've been on, no contest. I haven't had the privilege of going on Gwazi, but that's something to consider: IOA does not have a wooden coaster, if you like them a lot. Enjoy your trip!
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Po!nt of View: A different look at Roller Coasters.
http://www.crosswinds.net/~justmayntz/thrills/index.html