On an anon site I think checking back might be slightly better than betting with the 6 and BDFD/SD. I think both options are going to be good to some degree.
You are going to be able to call a bunch of turns and most opponents are going to bet QBBB on the turn. A 27/13 over 70 hands probably won't bet most of the combos that make a SD after he opens UTG on turns 2,3,T+. Also sets up a situation where the turn is 4-9 and he c/f a hand that would make us b/f flop or check back turn