I'd tend to guess, based on my knowledge of desktop graphics cards, that the 512MB 8800m GTS would be the better of the two. It would also be lighter and should in theory run cooler than a two card setup.
I haven't seen dual 8700s, but I have seen dual 8600s in several Toshibas. Either way, I'd want the single GPU for a variety of reasons... heat, battery life, compatibility, etc. Assuming the title fully supports an SLI setup, the two should be fairly evenly matched... but that's a fairly big assumption to make.
That Gateway with the 8800 GTS keeps tempting me (the one that is generally on sale at BestBuy for $1249) but there are just too many compromises with that system.