Either length should be fine. There have been lots of guys posting build reports on AR-15.com and nobody has had any function problems yet. The only issues have come from compatibility between these and certain other brands of lowers, most notably the PSA PA-10. I think PSA based their .308 stuff off the Armalite pattern, and these are DPMS pattern. Of course with AR-10s, I think it is best to stick with uppers and lowers from the same place, just to be sure.
I think both of these also lack the serrations on the bolt carrier, so the forward assist won't work. However, that's not really a huge issue. I've never had to use the thing on an AR, and if your gun is so dirty that you need it, you probably should just clean it. If you must have it, you can always buy a different bolt carrier.