G3a3 is a beast if you are good w/ aim. lots of recoil and less ammo in clip but damage is high. if u are a medium to long range gunner, this is a nice one to use.
F2000 is very similar to AEK imo. high fire rate (with little recoil) for the close-medium quarter combat. same ammo in clip as AEK.
Try the G3a3 first since you've mastered the AEK.