Sorry, I've never shot with the 75mm f/1.8 for any length of time (I've handled it - but that was a few years ago). However, I know people who have and were/are simply delighted. The lens is of course smaller and lighter than the 90mm f/2, but the latter has weather sealing (not that I had the body to match it, mind), and I feel a 150mm-e would most probably be too tight for me (the 135mm-e is a bit tighter than what I seem to pre-visualise, but by a negligible amount; I'll get used to it quickly).
Another thought: I don't really like the size of the 12-40mm f/2.8 on the E-M5 III, and the 75mm f/1.8's is almost the same, so ... On the other hand, its weight is just a tad above the 12-45mm's - which is a great lens in use*; though also more compact, which may be important. The 12-40mm f/2.8 always feels nose-heavy - not only because of its weight, but also because of the way it's balanced internally; lots of weight to the front, it appears ...**
Anyhow, if you want to stick with
, the 75mm is a no-brainer because it's such a great performer; noone I know of disagrees. For me, though,
is strictly the *compact* system, and I own and love the Sigma 60mm f/2.8 - in spite of its seemingly frugal specs and low price, it's a fantastic lens, and the best cure against lusting after the 75mm IMO.
means - to me - that I can cover everything up to 600mm-e with comparatively little bulk (the whole system fits into one medium-sized backpack!). The Fujifilm stuff is more about quality and results, a field where, in spite of all of its merits,
has its limitations. The 90mm f/2 fits that purpose perfectly - all the more since it's far less uncomfortable in use than I feared it might be (see **). That's also a testament to the clever design of the X-E3 - it's really small, but handles well with bigger lenses (better than the already nice GX9; the jury's still out as to how it compares to the E-M5 III***).
(I went OT a couple of times; I've put that stuff into footnotes to unclutter things; I hope it works.)
M.
*OT: Tthe 12-45mm f/4 is by far the best match to the smaller (and lighter) E-M5 III body. I kept the 12-40mm for low-light reportage, something it excels at - but the 12-45mm lives on the E-M5 III now (sometimes, the Sigma 30mm f/1.4 mu43:!) and 60mm f/2.8 get some use). And just to repeat that (I said it before): The 12-45mm f/4 *beats* the 18-55mm f/2.8-4 - in all respects, range, optics, ease of use ... and that's a fantastic achievement, as everyone who has handled the Fuji zoom will understand ...
**OT (slightly): It was a real eye-opener to handle the 12-100mm f/4 which, while a lot bigger and heavier, feels actually better balanced and less of a burden (that was on the E-M10, no less - though with the additional grip). The 90mm f/2 reminds me a lot of that!
***OT: I'm contemplating getting the grip for the E-M5 III because of all this - but may not even need the one I already ordered for the X-E3. I'll keep it anyway - all additional grips I own (several!) sometimes come in handy. Though I have to say it here and now: The worst(!) additional grip is the Leica one for the M10. I know what they thought - but the benefits are minor, and the downsides are considerable. The best additional grip I've ever seen and owned, on the other hand, was the one for the Olympus E-M10 ... the only camera I ever really regretted selling; it was such a well thought-out product. Yes, the GX9 and of course the E-M5 III take (slightly) better pictures and offer more, but the E-M10 was no slouch and handled like a dream when "gripped".