Unfortunately, I suspect the answer is “it depends”, as it will depend on a combination of your personal preferences and also the tubes you’re using.
@bpcarb and I were discussing this a few weeks ago when I was comparing 100 and 300 on my Nautilus (different amp, but closer family connection to the Forge than the Pendant). At the time, I was enthralled with how exciting the 300 ohm output sounded on rock music, using my Verite. But I neglected to post later that after the excitement wore off, it was just too much, and I reverted to using the 100 ohm output.
I’m using 5AR4 rectifier tubes, which have an aggressive quality to them, and in combination with the 300 ohm output, it was like watching a TV set to ‘vivid’. It’s really impressive because you are bombarded with so much detail, and bass lines are locked in. But just like tuning the settings on my TV to make it more pleasing for extended viewing, I came to realize that I couldn’t listen to the 300 ohm output for hours on end, whereas I can do that using the 100, which gives me a more liquid presentation that seems softer in direct comparison to the 300 when I switch over, but eventually sounds more natural to me.
The same effect can be heard when using the Aeolus, but maybe because it’s less resolving than the Verite, I can have more extended 300 ohm listening sessions with the Aeolus when I’m seeking some extra excitement while I’m listening to Tool.
So for my personal preferences, with my tubes, I would choose 100 ohms for ZMF, but YMMV.