The 550 Ti will be a tiny bit faster. Probably just a couple FPS in most games.
On the other hand, a pair of 550 Tis use a lot more power than a 6950, and will require a better power supply. Micro stuttering is also a potential issue with the SLI setup, and not all games respond well to SLI right after release.
You also get a relatively cheap upgrade path with the 6950, as you can add a second for CF later, while upgrading from the 550 Ti SLI setup will require buying a much more expensive card. If you're not planning on upgrading in the next year or two, though, that's not much of an issue.
Basically, I don't think it's worth it to SLI low end cards to get a 5% performance advantage over a single mid-range card that costs the same.