Here's an interesting question. Say the GOP nets 5 seats, leading to what looks like a 50-50 senate. In true reality, this is a 50-48-2 senate (with 2 independents who caucus with Democrats). So, which party gets to claim the title of "majority party"? I can see arguments for both.
The Democrats, of course. The President of the Senate is a Democrat.
Same thing happened after 2000 when the Senate was split 50-50, and the new President of the Senate was a Republican. (Until Jeffords defected, that is.)