It'll also come down to your preference, and how each gun feels in your hands. Also, you said you were mostly at an indoor field. will it be mostly CQB? If so, the SCAR's folding and adjustable stock is tough to beat.
If there is a way for you to try out other people's guns at your neaby airsoft place, try to get your hands on one of each gun, and see which feels better for you. Most airsofters are pretty good about letting people try their guns out, as the more airsofters we get the better.
as for the availability of each, your best bet is to get age verified, which i found to actually be a very easy and painless process, and then look in either the AV retailer section, or the classifieds. I actually just bought a SCAR yesterday, i'm just waiting to get ahold of it. Hell, if i see you at a game in the city, i'll let you give it a try.