AR15 is definitely cheaper to shoot. Won't get 'dirty' at the range. Military has now used them for over 40 years. Tons of extras and add-ons with new stuff every day. My son (82nd airborne) puts his butt on the line with an M4 - no complaints. Then again, he's backed up with an M249 (SAW) and a 240B (FN .30 cal).
Socom II is a short, neat variation of the MIA. Personally, I have more 'fun' shooting the .30 cals. Whatever floats your boat. It's all relative.