From experience, I'd say the Ghost M4 is the better choice for sparring. Its low freeze and high damage make it quite powerful in capable (and incapable) hands. In comparison, the BAR is a great choice if you intend to base or pk because of its fire rate and high damage. The BAR can also be strong in spar if you're skilled enough to play with its low freeze (essentially comes down to you not spamming and timing your shots more carefully) although I will mention that players can tend to get irritated if you use the gun in spars (but who really cares about all that )