I got my pair of Ollo S5X last week and was eager to compare them. I have had about 10 hours of listening to both.
Both were again driven by my Apollo Twins HP-Output.
Comfort
Material is the same with both. Clamping force has decreased a little with the S5X. The ear pad diameter on the other hand has increased and fits my ear slightly better.
The headband has been redesigned completely and is not self-adjusting anymore. That lowers the pressure of the headband against the head.
I find the S5X to be even more comfortable than the S4X.
Evaluation Tracks
Same as in my post Ollo S4X vs. AA Hi-X60...
Ollo S4X / Austrian Audio Hi-X60
...plus a few others. I am still working on a list of reference tracks...
Preamble
I love the S4X...but I feared their slightly foward sounding upper bass / lower mids might lead to wrong mix decisions. So I ordered the S5X when I got the S4X.
As has been said about FR-measurements - comparing different manufacturers measurements might not help a lot. In this case they are of course both by made by Ollo on the same test rig.
So they are very comparable....Thanks to Ozonepaul for making me aware of that pdf.
Ollo Measurements
It shows the S5X closer to a flat line in bass and lower mids. Also treble looks promising.
One big difference between both are the angled drivers in the S5X.
Rok (the founder) said, all changes were done to accomodate the different target curve for immersive audio mixing though stereo mixing on the S5X is no problem at all.
Sound
Both have the same Ollo House-Sound. No surprises at all.
The S5X sound very much as I expected them to after seeing the FR-Measurement - and more.
Bass / lower mids are more to my likening now, though a slight lift to the sub region makes them even more fun (I tried a +2dB shelve at 60 Hz).
Details are more easily detectable - even with the bass eq'd. Treble is great now. More air, more presence without being too loud or harsh.
Where I thought I might overcompensate with the S4X I don't think I will with these.
The biggest difference between both Ollos is depth. This might also be due to the angled drivers. Whereas the S4X are presenting everything quite upfront (like in a straight line in front of me), the S5X present instruments/voices in a more spatial manner. This makes the S5X slightly less engaging and a little less fun to listen to but for mixing I find that to be of invaluable importance for judging reverbs for example.
Conclusion
There are differences between both headphones. If you are doing immersive audio mixing the S5X seem to be the way to go. Up to now I haven't looked into that very much. Stereo is still enough of a challenge. Should you upgrade to the S5X?...I would at least recomment to test them.
Both are a joy to mix with. Had I not listened to the S5X I would not miss anything at all. But I am glad I did. Even though I sent the S4X back I may order a pair next year just because the differences between them are still big enough to justify having both..

....But before I do so I will have a listen to the Neumann NDH-30 as they might bring something to the table I haven't heard or thought of yet.
I think I understand Rok's reasoning for having both in production - the S4X for stereo and the S5X for immersive audio mixing. But personally the S5X suit me better and strengthen my confidence in mix decisions even more than the S4X.
Best regards
Daniel