What kind of shooting do you do? Do you plan on building it, or buying it complete?
If you want to shoot targets at longer distances with pretty good accuracy (100-200 yards or more), go with 5.56. Just make sure to get it chambered in 5.56, not .223. They are both basically the same round, but you cannot shoot 5.56 out of a .223 due to 5.56 being a hotter load. You can shoot .223 out of a 5.56 though.
If you just want to send rounds down range at shorter distances, go with the 9mm or .22 LR (if you want it really cheap.).
Hope this helps.