Until this year, my son played in the 6th grade year in the area. If you've never played AAU before, here are some things you need to understand. If your kid is not one of the top 7 or 8 players on the squad, they may not play much. These teams play to win not always to develop all players on the squad. Most of these teams practice almost year around. The AAU scheduling is a nightmare (you will be in tournaments and leagues that don't publish your game schedule until Thursday two days before game time.) Parent culture can be horrendous -- lots of yelling at refs and kids (that's probably everywhere these days).
I'd move to 6th grade to try to get as much playing time as possible. I also suggest you visit some teams for a practice during the winter. That will be the way to figure out if the squad will be a good fit for your kid. Contact these two organizations and tell them you're interested in visiting a practice and learning more about their program even though you may not really be looking to play until the spring.
When you visit, focus on these things: 1) coaching style and philosophy of the club; 2) is it parent coached (this often/always creates issues of favoritism for coach' kid and kid's friends for playing time); 3) how much tournament and out of town travel do you want to do? 4) where will my kid fit in with this squad?
With those two questions in mind, I'd check out these two organizations that I've been relatively impressed by:
NOVA 94
http://www.nova94feet.com/
MADE basketball
http://www.madesportsunlimited.org/ (but that would require you driving to MoCo).