As others have mentioned, AC for sure. it's less efficient.. but useful and practical for so much more than just charging your batteries. You have to think about this investment over the period of "your lifetime", and how much technology changes in batteries over time. 12v, 24v, 48v, whats next? will your DC generator be able to do whatever is next?
AC is here for the forseeable future, and the resitance to change from 120/240 in the US means it will likely be around for the rest of yours and my lifetimes. You can always buy a new battery charger that charges from AC because it wouldn't be dependent on one particular brand, or dependent on the company still supporting your generator model. (ie, to sell you a conversion kit from 48v to.. whatever.. 96v.. etc.
However, battery chargers for whatever the new voltage standard is, will be readily available to buy from multiple suppliers/manufacturers that all take an AC source.
I'd take the hit in efficiency, for the future upgradability/expandability.