Brendan - it depends on the position you're welding, and the rods you are using. If they are 2.5mm 6013's then you probably want the upper range - near 90v (which the box will probably say). But if you were using cellulosics it might be half that amperage. With 7014's etc, it might be a lot more.
Either 2.5 or 3.2 will be fine. TBH mate, the best way to find out the correct amps is to have a go and see what works best. You could also try looking here for some clues.