I would go with the 16". It will be slightly lighter than the 18" and approx. 50 fps slower.
The 6.8 is designed to perform well out of short barrels. There is not much difference between these two so really it's just a personal preference.

 

I have a PSA 16 in., a Stag 7h 20.77 in. and I just got an ARP 18 in. for a friend. I use the Stag for hunting deer from a blind here in MI. and the PSA for hunting hogs and deer in Texas. After shooting and handling the ARP 18 in. I'm thinking of selling the Stag and getting one for my do everythig upper.

 

At those ranges, the 16" barrel would be more than sufficient and you would not gain much of anything by going with the 18". That being said, I still like an 18" target crown...you only really add about 1/2" on overall length (no muzzle device) and get the additional velocity. The additional weight is the trade off if you're trying to keep things ultralight. With hogs in the picture though, I could see the benefit of a good flash suppressor, so I'd lean towards the 16" anyhow. Always hate being blinded by the flash and needing to let my eyes re-adjust...while attempting to track a critter I just shot.

All things considered...I'd go with the 16" and not think twice about it. Two inches seems much larger in theory than it is in reality. Take a look at a ruler and it puts it back in perspective.

Buy both

 

The answer is obvious.................you need 2 6.8s! Haha!

Your specs are very similiar to the upper I have for my daughter to use hunting. 16 in. bushmaster upper and redfield 2-7. Ours shoots 120 sst's quite well. Not so good with the 140 bergers tho.
I have another 6.8 for me with a vortex on it. I like the vortex better.
I want to use the 140 bergers for her to take a moose, so I just ordered an 18 in. upper from PSA. We'll see how that works. I would also like to get away from the extra noise the flash hider adds to each shot. Some 18s come with a target crown.

 

16" here with the Yankee Hill Ti can on the end of it still 24 inches long.

 

If you can't decide between 16" and 18" why don't you go for the 18" then get it shortened to 17".......best of both worlds!
Jerry, (UK)

 

If you go with an 18 target crown it will be about the same weight as a 16" with a muzzle break plus you get a tad more FPS.

 

Thats what I did.

 

For the range you specified 16" is fine. But, you can do the target crown thing too.

 

When I initially ordered my Bison upper I ordered the 18" barrel. I did a lot of research and found that the added velocity significant when comparing 16" to 18" in 6.8. I changed my order to 16". I'm glad I did, as it handles beautifully. I would suggest the 16" but thats my opinion.