I wasn't aware there was an increase in DR in the 6D? Not beyond marginal, at least?
For the OP's needs the 5d2 sounds perfect and I think the 'raft' of extra features in the 6D (in this instance) are not exactly essential. Nice to have but are they really useful? I would suggest the extra ISO capacity is the only enticement... And maybe just enough to justify the extra... The decision is not as clear cut as some would make out though, IMO