I imagine if your team can hold out against a baron'd up enemy until the buff expires then the 5th dragon buff would win simply because its a permanent buff as opposed to a timed buff. This may also be the case if you don't think you could win a fight against the enemy at baron.
If you think you can beat them at baron then your best bet would be to fight at baron obviously and get the dragon later.
At the end of the day I think the safer bet would be to just take the dragon and turtle until they lose baron buff.
There are a lot of variables that would go into that choice though such as team comps and the number of towers you have left.