I'm considering a dedicated 9mm AR for home defense (I already have a RRA AR15, Armalite AR10 and a POF 308 lower for a future project).

I want something short and quick to handle with options for a flashlight and/or illuminated optics/laser.  So I'm thinking an A4 w/ collapsible stock and a quad rail.  

The question is, should I buy a true 9mm lower with a 9mm upper, or a regular 223 lower with a magblock and a 9mm upper?

I'm leaning toward a true 9mm lower, becasue I already have the other lowers.  Is there a reason I wouldn't want a dedicated 9mm lower?  I'm considering a RRA, but are there any other makers I should consider?

Thanks

You'll probably get more response in the Pistol Cal section, but I say go for the dedicated lower.  Others will say get the .223 lower and mag block, but honestly, how often do you swap uppers around?

I have a dedicated Glock lower from OlyArms and am waiting on paperwork to complete my SBR .40 S&W upper for it.  If you're wanting short, you might want to check out building an SBR.  I really like the Glock lower, since I carry a Glock, my SBR and carry pistol will have interchangeable mags.