Neither i'd say.
As long it's pretty certain evaporation are the cause of the dryness of the cell (not spill), you should add
only water.
Top up the rest of the cells with water.
The cell that was dry will probably need more charge/discharge cycles to wake up than the rest of the cells, and it's not certain it will recover, due to exposure of air to the plates. (If it recovers, it will probably not have the same capacity as the others... )
You could build 2 of those 12V desulfators and connect one on each half of the 24V bank.
(I have no opinion on desulfators, so others might chime in.
I would try charging and discharging the bank several times to wake it up.
)
Edit:
Get yourself a good meter to measure Specific Gravity (SG), and write down readings of each cell after each charge, And discharge... That way, you'll get a really good idea of the state of each cell.