AG3 wins by a mile in my opinion. So many improvements over AG2:
-Much more customization for the girls (face, voice, traits, etc.)
-Up to five girls at a time
-The girls talk to you and interact with each other (and even kiss)
-Better graphics
-No stupid 9-minute time limit
-She won't get upset if either of you cum too many times
-"Perpetual orgasm" mode
-Easier to control the action with the scroll wheel
-Easier camera control
-Anal
-More positions
-You can "break" the girl and make her lewd all the time
-Horny girls attack you and even rape you in your sleep
-Clothing is much more customizable
-You can easily create your own clothing mods by working with .tga files
-Jealous catfights
-It doesn't rain every 10 minutes
-Bukakke!
Though this is what I like better about AG2:
-You can do her on a table or a bench
-Save your scenes as a "movie"
I can't wait to see what Illusion does with AG4, though my aging laptop probably can't handle it.