I haven't played many FPS and only play them vs the AI.

Ghost Recon felt fairly realistic, but maybe too much so - you often get killed by one shot by someone you don't even see; realistic but not fun. A lot of save and reload; and later taking advantage of previous run throughs of the game so you know where the enemy were. Conversely, you usually kill without being seen - to do otherwise invites disaster. Plus the whole game feels rather soulless.

I much preferred Vietcong. By varying the difficulty level you get go anywhere between Ghost Recon (or tougher) and a more conventional, "shoot me, I can take it" game. I found a happy middle where you could risk a firefight but not feel like the Terminator. The feel of the authentic weapons and taking advantage of cover was great too. Plus I really enjoyed the setting, story and variety of missions.

Both games have a lot of openness and a squad, which I like rather than being sole protaganist moving down the rails. Haven't tried Operation Flashpoint yet but it sounds my cup of tea.