amplifier impedance

"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.

Please clarify what you mean by "unstable." The output impedance of a tube amp is generally higher than a solid state amp but I don't think it can accurately described as being unstable.

The 4 ohm tap will provide less power than the 8 ohm tap so if you listen at loud volumes that may be a factor. There is no danger to the amp or speakers.

Mostly likely the 8 ohm would be best. With most tube amps the actual impedance is rather unstable, one reason most people like solid state units. As for safe, hard to tell without knowing the make and model of speakers being used.