Which is more efficient and requires less power?

The question of imedance drop is important. The 6 ohm speaker dips to 3 ohms. I don't know about the 4 ohm nominal speaker, which conceivably could dip lower than 3 ohms.

The idea is to use a PP triode tube amp with a power output of 50wpc or less.

If one were to have a tube amp with say an output impedence peak of 3.5 ohms at 200 hz and speakers which have an impedence droop to 3.5 ohms at 200 hz would the two cancel the potential negative effect of either, everything else being equal?

Looking at this from the other end, how loudly do you like to listen to music? How large is your room? These answers may have more impact than going strictly by speaker specs.

I auditioned a pair of huge JM Lab Electra 946 with a 3.5W ASL 2A3 amp. It worked great as long as you didn't turn the volume up too much. As an owner of a McIntosh amp with power meters, I can tell you that most of the time, I use less than 5W of power. Maybe you are the same way?

I sit 8 feet from the loudspeakers in a 19x16x8 foot room. I listen
around 89-90db, and occasionally crank to ~101db. Classical to jazz to
heavy metal. At the loudest volume, I'm using around 32 watts. My
present speakers are the 89db/1w/1m in the example above.

It seems 91db, 8 ohm speakers would be a more significant change from
what I presently own in terms of efficiency and driving ease than would
94db, 4 ohm speakers.

Depends on the amp also.
Most solid state amps will double power output when going from 8 ohms to 4. While most tube amps won't increase output ratings at all.
Try this chart also:
@8 ohms for 89db speaker
1 watt = 89db output
2 watts = 92db
4 watts = 95db
8 watts = 98db
16 watts = 101db
and so on.....
every increase of 10db will give you a perceived doulbing of volume.
Compare your amp rating for 4, 6 & 8ohms if you can.
This will let you know what the actual output will be.