I'd go for 1/72/20mm scale. There are a HUGE number of available minis. I play marines too and am using Italeri's modern US infantry for my normal infantry squads, Caesar's modern US elite force for my Force Recon marines. The Italeri models have slightly outdated equipment, and the Caesar ones have state of the art stuff. That makes it much easier to tell who's who. I spruced up the squads with OKB Grigorov brass etched M14's in place of the designated marksman rifle in the BFE book... Mostly because I like M14's. I used Dragon 1/72 scale HMMWV's and a Hobbycraft 1/72 AH-1z, though I wouldn't recommend the hobbycraft kit. It's out of scale and a bit of a pain to assemble as it's just a modded version of one of their other AH-1 kits and they expect you to trim off huge pieces of plastic, and even the pieces that normally are supposed to fit together require a good amount of putty to fill in...
The biggest advantage of 1/72 is cost. All that cost me just over $50 (not counting shipping) and I have 60-some infantry models left over!