"Would one be more optimal than the other, or more "safe" for the amp or speakers?"
When in doubt, in general, the 4 ohm would be the "safest" bet. If 6.5 is the minimum, you should be fine with the 8 and while you could use either, I think the 8 will sound the best. Go with the advice you were given and try both. It's about your ears.
When in doubt, in general, the 4 ohm would be the "safest" bet. If 6.5 is the minimum, you should be fine with the 8 and while you could use either, I think the 8 will sound the best. Go with the advice you were given and try both. It's about your ears.