A few other points to consider...
1) Because of the additional stress on the bolt by the more powerful cartridge, 7.62x39 ARs tend to eat bolts faster. And the shorter the barrel, the more parts breakage you're likely to experience. If you get an SBR below 16", you better keep a few spare bolts on hand whenever you shoot. If you get a 20" you can probably go much longer without breakage.
2) AFAIK there are no chrome lined 7.62x39 AR15/M16 barrels - stainless or chrome-moly. If you use corrosive ammo, you can ruin the upper quickly if you don't clean properly (and quickly) after each outing.
3)Parts availability - as you probably know there are far fewer places to get 7.62x39 AR15 parts. That may not be a big deal to you now, but when/if you break something it could become a huge issue. For this reason I like to keep 7.62x39 in my AKs - parts are relatively cheap and plentiful compared to 7.62x39 AR parts.