I only have one complete RRA 9mm flat-top so take this FWIW.
Immediately upon purchasing was less than overwhelmed. I had initially purchased it to replace my Colt 6920 in the squad car with me but then realized, "The purpose of a handgun is to fight your way to a rifle" and I'm pretty sure whoever coined the phrase didn't mean a pistol caliber rifle/carbine. I can also make a rifle caliber shoot short but it's a bit more difficult to make a pistol caliber shoot long.
Another thing that peeved me was the cost of mags and their limited avalibility.
After not being able to sell the thing I decided I'd accept it and enjoy it. Since that time I've come to thoroughly enjoy and like it. I found cheap Uzi mags at
Cole Distributing and had a friend who was a machinist in another life time cut them.
I have yet to qualify with the 9 but will do so along with my other ARs so I can use whatever suits my fancy on a given shift.
My opinion is, if this is your first and only AR, get a 5.56 and then look to a 9 in the future. If you're pleased with your 5.56 collection, then definitely get a 9. They're great fun to shoot, are very accurate (in my experience) to at least 100 yards with WWB Value Pack ammo, and operate essentially the same as their rifle caliber brethren.
Long-winded and but one opinion, but since you asked,
Scott