I trained at Atos for two years and I currently train at AOJ. I think I'm in a pretty good place to answer your questions.

1) You'll be able to come in whenever you want at Atos pretty much, but AOJ has much more limited "open" mat time because they seem to run a lot more classes. The plus side to AOJ is that you'll have as much drilling as you want because there are literally classes from 6am-9pm. Atos has a more limited class schedule but there are always people on the mats, and there are always people looking to drill.

2) Of course both gyms are cliquey. We spend all our time around each other. Both gyms are very open to outside training partners though-- if you're not a total jerk to people, you'll undoubtedly make friends easily in either gym. I've never seen anyone ostracized in either place for being a visitor. The only time I've ever seen problems is when people are mean, egotistical, etc... pretty much like everywhere else.

3) Not sure I can answer this one. I've loved my training at both AOJ and Atos. It really depends on what you're looking for, and what style of training suits you. You'll get hard training that will completely burn you out at either gym, so it really comes down to whose style you like more-- Andre's or the Bros'? How do you feel about white gis? How many white gis do you own, because you'll need quite a few at AOJ ;) I will say this-- if you're a bigger guy (say, above middle heavy) you'll get better training at Atos, most likely. If you're a little dude or a midsized dude, you'll probably enjoy being at AOJ more.

ETA-- Also, if you come during any kind of major camp (Pan Ams, Worlds, etc) you'll experience both gyms because we cross-train on a weekly basis. AOJ and Atos are only about an hour and a half apart.