I'm with almarg on this one - and kudos to him (her? sorry almarg!) for doing the research.
The fact that the minimum speaker impedance is 6 ohms, combined with the output impedance of the ARC amp of about 1 ohm, brings us up to 7 ohms total (roughly). For a "set it and forget it" approach, I'd go with the 8 ohm tap.
That said, as seikosha pointed out, it really is a personal taste question and it's up to your own ears to decide.
If you can't hear the difference, then take it as a sign that you may be spending too much money on audio gear - as well all are. ;-)