It depends on which year you're talking about. 2000 - Dems. 1994 - GOP. Since this is not a Presidential election, that will favor the Pubbies a bit.
It's still too early for us to know what will occur, no matter what anyone says. There are a ton of close races, and a national trend in either direction could make for a blowout for one party. Or we could basically have local races and little change. We shall see. If you look at just poll numbers, I think the Pubbies are in better shape than the Dems. That would mean something if the elections were held today. :-)
You've got that backwards. The "out" party typically does better in non-Presidential years.