Different upper receiver. A2 has carrier handle built in A4 has a top rail. A4 would be better.
As far as buy or build....
You can build a better rifle. You can build rifle cheaper. You cannot build a better rifle cheaper.
The best deal right now is BCM as far as cost for quality. Don't let people fool you, not all AR's are built equally.
"Throughout history, it has been the inaction of those who could have acted; the indifference of those who should have known better; the silence of the voice of justice when it mattered most; that has made it possible for evil to triumph."
-Haile Selassie I