As someone mentioned, it depends on the chargers.
What is the rate of charge on the generator? What would the rate of charge be on the charger you would use? You may find that a good charger will only take a couple of hours, where as the generator would require longer, if it's a lower charge rate. I imagine you are asking so as to figure which would cost you less in gas to charge the batteries.
If you already have a battery charger, take it with you next time, and do a little testing to see what results you get...testing the batteries every 30 minutes or so, to see when they are fully charged. Then charge with the generator and see how long that takes......
And finally, check to see how much fuel you use in each instance, because you may find that the generator is running at a lower load with it's charger, so uses less fuel than when the battery charger is plugged into the 110VAC.
Good luck:)