On the issue of Performance, realism, and cost.
WARNING!! ADVICE IS FROM A NEWBIE!!!
Tokyo Marui (considered the best Airsoft company by most) - M4A1. I have heard very little in terms of problems for this rifle, with the expection of the "infamous" 'barrel-wobble'. This problem will cause the unit to fire off target but it is fixable. The TM - M4A1 has an amazing amount of upgrades available to it.
G&P - M4A1 . These are (to my knowledge) mostly metal and would provide the most realism in the area of weight. The only problem (that others have pointed out) is the need of a more powerful battery to turn the upgrades that are generally pre-installed.
Keep in mind that most fields impose a 400 FPS limit for all AEGS and usually a 500 FPS limit for Snipers.
Welcome to the wonderful world of Airsoft.