Simply regarding the opening: 6...Bb4 is considered Black's safest method of equalising, and indeed scores considerably better than the alternatives - 6...Bb4 scores 47%, 6...Be7 scores 42%, and 6...Nc6 scores 39%.
This means that, unfortunately, 7.a3 doesn't give white the better game. Black scores 56% (!) after 7...Bxc3. Play continues 8.bxc3 0-0 9.Bd3 dxc4 10.Bxc4 Qc7! and White must spend time guarding the bishop, and is generally a tempo or two short of setting up his desired central position.
However, White doesn't have to play 7.a3, and the difference will explain why 6...Bb4 is considered better than 6...Be7. After 6...Be7, White's best try for an advantage is with 7.cxd5 - indeed this scores an extremely good 63% for White, play will usually continue 7...Nxd5 8.Bd3 Nc6 9.0-0 0-0 etc. But if White plays the same in the 6...Bb4 variation - 7.exd5 Nxd5, suddenly Black is attacking c3 twice and White must expend a tempo doing something about it - this is the crucial difference.
It's that time of year again, and the Portsmouth District League has begun. Last night, I played in a match against Portsmouth University on Board 2- we won the match 3.5 - 2.5, so a good start for us. Before commenting on the game, please do note this is a Division 3 game in the league. (I sometimes play in the second division as well.)
My opponent was rated 179 five years ago, I can only assume that he has just returned to the chess world as otherwise I would have had roughly 0% chance of defeating him! The time controls in Division 3 are pretty fast, but are considered standard-play, just, 30 moves in an hour and then +15 minutes to finish.