Load times are ANNOYINGLY LONG in BF3 (but work well for bathroom breaks). It would help you out there, as others have said. However, if it is your only drive, 300GB isn't all that much space anymore if you're installing lots of games, music, videos, etc.
I would size it based on your space needs, not game load time improvements. If you don't need the space, go with the faster drive. If you plan to have a lot of games/music/videos on your computer, go with the larger drive. It really is that simple.