4 ohm load vs 8 ohm load? What is prefered?

If they are equal in effencey then there be no difference at the same level, the 4 ohm would have a little more power available and play a couple of DB louder. Neither is preferred in itself. A transistor amp will try to double its output when the impedance is halved. Most don't have the capability, the Mac should be producing 240 instead of 200 but that is not bad. So with lesser quality transistors low impedance's should be avoided, even high quality ones can have problems with VERY low ones, say under one ohm. Since tubes usually have transformers to match their impedance they do not operate in this manner. The impedance derives from the particular design of the speaker and neither is preferred in itself, they are a consequence and not a design goal except to make them usable by the amplifiers in their price class. Typically, the more drivers the lower the impedance.

The way I understand it is given the same sensitivity the amp would have to double down into 4 ohms for the 4 ohm speaker to play as loud as the 8 ohm speaker. The MA6500 almost gets there so it would be very close. I doubt if anyone would even notice.

With 4ohm you gain 15.7% of perceived loudness but loose almost 100% of damping factor (120 vs. 230). Choose speakers that sound better to you - forget numbers.

Perceived Loudness = k^(1/3.5) where k is a ratio of power.

Kijanki, I doubt that anyone can pinpoint the difference between a damping factor of 120 vs. 230.
Only poorly designed, very boomy speakers require a damping factor above 20. These are just numbers, as you pointed out.
The OP should decide based on listening tests.

Casouza - I agree. I believe that great specifications are often a contrary indicator of sound quality. I would, for instance, avoid amps with thd=0.00001% at any cost.

Higher df might be of some value since xover inductor in series with the woofer is in order of 0.08 ohm limiting df already to about 100 resulting in total df=50. DF gets much worse with frequency and might get worse for small signals. It is all, most likely, speaker design dependent.