I haven't tried Spectre Ops yet, but I am looking forward to trying it when I get a chance. As to Sangin, I think it works great and feels the closest to reality when there are less than 30-40 minis on the board and when you have fewer than four players, but when you play it in a large group (convention style) or with too many models, you can get pretty bogged down. Sangin is best played with a couple players who know the rules, which means that there will be little interruption of the game flow. It's definitely one of those systems that can put people off if they only play it once or twice and have to constantly be looking things up.
Bottom line:
If you are looking for 1-2 squads worth of men, go with Sangin. If you want a platoon, try either Spectre or Force on Force.