Which is the better choice, AC output UPS or DC output UPS?

It depends on the output capacity in watts. If the load is less than 200 watts, DC output UPS systems are about half the size, weight and cost substantially less than an AC output UPS. This is because an expensive DC to AC inverter circuit can be eliminated and there is no power conversion loss associated with DC to AC inversion circuit, resulting in substantial battery size reduction as well.
However, if the load is more than 400 watts, then the size/weight/cost penalty of AC output UPS is offset by the need for expensive, continuous-duty high powered AC to DC converter in a high capacity DC output UPS. If the UPS must power a load of 800 watts or more, AC output UPS is by far the smaller, lighter, more energy and cost effective solution.