I have to disagree with the above statement, perfectly identical bases are not perfectly balanced.
A human ideal base would be something like a long corridor wide enough for three turrets, with enough space for players to maneuver around.
An Alien base would be something with.. alot of corners, places to hide tubes, jagged ceilings for trappers, a corner to hide the om in, and outlying little nooks and crannies for hidden eggs, and lots more space to maneuver around in.
ATSC does a good job of blending these characteristics into two identical bases, but no map will EVER be perfectly balanced, no matter how well constructed it is.
It does depends alot more on the players, if aliens can keep a good offensive going, it is possible that the construction of their base never plays a role in their victory. Same with humans.. but less likely.