SLI does get microstuttering. If you search, you'll find plenty of examples. However, I have used both a lot, and didn't have microstuttering from either.
Most the time there is, in the cases of what I've seen from reviews and users, it's game related. At least in the case of SLI.
I've seen users come here with microstuttering, and some fix it due to some software of setting problem. This seems to be more common with crossfire setups.