Allied Assault is over rated, multi is fine as long as you dont mind someone letting off a rocket in your face every 30 seconds, dont know about the AI being better, the friendlies in AA were about as dumb as they get. Enemy AI is good in both versions.
Frontline has much better levels imo, Arnhem Knights, Rough landing are fantastic, as is Omaha which is far more interactive than AA\'s mad dash for cover.
Frontline maybe a little short but its still a great game, well worth buying.