Better performance of course. Stuff like higher fps, ability to run most games at FULL/MAX settings (depending on which video cards you use in SLI), better graphical display, and also better benchmarking scores.
But it's not that great of an idea to use SLI.
Let's say you want to use two 6800GTs in SLI, the total can come up to about $600 for both cards. But you can get a single 7800GT, for about $400, and still outperform both 6800GTs. Heck, get a 7800GTX for $500 and easily outperform both 6800GTs by 20fps+.
So yeah, the only way to have the best price/performance ratio in SLI is by getting 7800GTXs in SLI. Otherwise, getting a single card of the higher quality is better.