Neither are ideal because most "welding wire" is fairly low resistance so needs quite high current at low voltage to get it hot. You might get away with the 3A one but I think you'll need more current (or different wire).
Most of the discussion about powering hot wire cutters is in the Foamies (Scratchbuilt) forum. E.g. http://www.rcgroups.com/forums/showthread.php?t=1725654