12v is a lot more managable, I think the regulator of a standard arduino accepts up to 17v's so a 12v supply would be perfect as it would then supply you both
your 3v 5v rails and 5v logic pins.. but, you also have a high power source 12v with a high current you can use the lower 3-5v to switch on and off with power mosfets
and Power Transistors.
[edit] just noticed it's an AC out, even better, use 2 diodes to rectifiy the signal to dc, and you'll have something more like 9-10volts DC out, almost perfect voltage for a typical
arduino board (2 diodes, 20cents each..)
Any voltage you need, just buy a regulator for that voltage?...
Or am i missing it?