The 5.1 is 5.1 inches long, and the 4.3 is 4.3 inches long I think. 5.1 is more for accuracy and precision, while 4.3 is more for combat and tactical situations. If you're going into airsoft practical shooting, generally a 5.1 would be better. I've seen many more 5.1 hi-capas modded than 4.3s, but both guns have many after market parts.
Last edited by ShadowNet; April 1st, 2009 at 20:52..