I've always wanted a shorty DSA FAL OSW. I see that Atlantic is selling a DSA FAL pistol with a sig brace.
Can anyone tell me if these are reliable ? How's the recoil and reliability with that short barrel?

Should I go HK91/PTR pistol or DSA FAL ?

The DSA FALs are fantastic. If you want a shorty DSA OSW, I recommend doing it the right way and do a Form 1 for a SBR and stay away from the pistol version with the SIG Brace. With the DSA OSW, you are most likely going to shoulder-fire it only and there was a recent letter (Nov. 2014) from the ATF addressing this issue with the SIG Brace. As for recoil, as with any .308 SBR, you are going to have quite a bit of recoil. This is another reason to avoid the pistol version. You are going to want to shoulder fire this weapon and not shoot it unsupported. The FALs have an adjustable gas system which you can dial in to adjust to the ammunition used and the cycling.

I would also not recommend a PTR-91 pistol either. I have a PTR-91KP (16" barrel and retractable stock) and it does have some recoil to deal with. With any short barreled .308, you want one with a stock. Neither the DSA OSW or the PTR-91 is any where being light weapons. They are heavy.

I went with the 11 inch barrel SBR
2 ft collapsed
Any shorter than that isn't worth it. I'll get out to 500 yards

I got to shoot a full auto OSW at the ASC Zombie shoot. It was amazing.