I don't think anybody opts to go for private loans anymore. Last year some schools weren't up to date with the GradPlus stuff, some lots of students did private just because of that. This year, I think they're all on board for quick GradPlus processing, so that's no longer an issue.
It's possible you could get a lower interest rate with a private loan if your credit is really good, but that whole variable interest thing doesn't make it worth it for me.