Re: Which but stock for my new carbine build?
I don't personally care for the ACE stocks because I have found the quality to be hit and miss.
The UBR is one of the finest AR stocks on the market right now in my opinion. They are strong, solid, and unlike many other collapsible stock setups, they offer a consistent, firm cheekweld thanks to the fixed position upper half of the stock. Now, with that said, they are anything but light coming in at just over 2lbs with the stock, buffer and spring installed. This is nice when it comes to balancing out a rifle with a heavier barrel or one with a little more forward weight, but it might bother some people...particularly those looking for LIGHT weight.
Other options you should consider are: 1) the VLTOR EMOD stock (strong, locks up tight, and complete it weighs in at around 1.5lbs incl. the buffer, receiver extension, spring, etc.); and 2) the Magpul ACS (again, a strong, well-made, but lightweight collapsible stock that weighs in just a little less than the EMOD when fully equipped/installed on your rifle/carbine).
Here are links to both:
http://www.vltor.com/EMOD-New.html
http://store.magpul.com/product/MAG370/87
FYI, since you provided MidwayUSA links to the products you are considering...both of the stocks I suggested in addition to the UBR are carried by Midway.
Good luck!