Well, I didn't get in on the street debate, but I would like to point out that entry to Diagon Alley will be controlled just like it was (still is?) at Hogsmeade.
Also, I seem to remember that one of JKs requirements for Diagon Alley (this being 5 or so years ago) was that everyone goes through a brick tapping ceremony to get in (you know, 3rd brick from the left and something up...). I think it was Disney's plans that called for 4 simultaneous entry rooms to get people in faster because everyone had to go through that to be authentic?