If you already have the amp and speakers, just try it out and see what works best for your speakers and your taste in music.
Not sure if you have a frequency vs. impedance graph or not, but if you have one and see a big dip in impedance, I might give more thought to a 4 ohm tap if there isn't much sonic difference between the two.
But really, try each and go with what you prefer. If they sound the same, I'd go 4 ohm, since even with 6 ohm nominal impedance listed, there are often some dips in a freq vs. impedance plot.
Not sure if you have a frequency vs. impedance graph or not, but if you have one and see a big dip in impedance, I might give more thought to a 4 ohm tap if there isn't much sonic difference between the two.
But really, try each and go with what you prefer. If they sound the same, I'd go 4 ohm, since even with 6 ohm nominal impedance listed, there are often some dips in a freq vs. impedance plot.