As the title states, I have My eyes set on a Sig P516 AR Pistol with the Sig Arm Brace.  I'm having trouble deciding between the 10" barrel and the 7.5".  Both would suit my needs as a PDW. The 10" will give Me more velocity but the 7.5" more portability.  Is there an advantage to the longer 10" as far as reliability is concerned?  How has the P516s reputation been holding up from owners?  Any idea what the velocity of XM193 is out of a 7.5" and 10" Barrel?

My P516 has been very reliable. I went with the 7.5 inch version (with a Law Tactical Side folder on order) as I wanted something backpack small.

With the shorter barrel, I bought a bunch of Hornady TAP 223 Remington Ammo 55 Grain Urban V-Max for it.

I have been seriously considering a 10in, as I believe I could use M193 ammo and be okay. My 10 inch would be more all purpose / truck gun  (since it would be classified as a pistol) without the class 3 hassle.

I am considering Daniel Defense, Sig PM400 or one of those Radical Firearm versions. Right now I am leaning towards the PM400 without brace, so I can use the newer SBX brace on my 7.5 and use the SB15 on my P516.

Nice looking P516.  I'm leaning toward the 7.5" right now due to the compactness and the fact that I plan to use it as a 50 yard max PDW with a red dot.

Sounds like you and I have very similar projects in mind! I'm on the wait list at my local shop for when they get the 7.5" 516s in stock, as soon as they get one I'm picking it up. For the use you've described I think you'd be better served by the 7.5" barrel - more than capable accuracy and power wise at the ranges you've mentioned, and that much more compact than the 10" barrel.

There are several P516s on gunbroker right now with the 7.5".  Prices are $1450-1500.  Seems to be many more 7.5" than the 10" available right now.