I plan on using Olympus M4/3 when it best suits my needs and using Sony FF at 42MP when it best suits my needs. I like having 2 kits, it gives me lots of options. I rarely shoot needing high ISO, instead I have fast primes, so the ISO advantage of the Sony is lost on me. The MP advantage of the Sony really depends on what you are shooting.
For portraits, in both kits I have small primes and there is not a lot of difference in weight or volume if I plan on using a Sony kit of 35-55-85 or 17-25(or 30)-42.5 with Oly/Pany. For Sony I can add the Batis 135mm f2.8 or for Olympus the 75mm and though the Batis 135 is a bit larger and heavier than the Oly, it is still very manageable. Higher MP are of no real help here, but the couple of stops better IBIS in the Olympus EM1.3 has proven useful. The excellent Eye AF in the Sony does not cover the full screen of the larger MP bodies, many 42 or 61MP portrait shooters of Sony gear have gone/added back the 24MP A7III for this reason, as it has full screen Eye AF like the EM1.3 has.
For sports/action/nature I use the Oly 7-14mm, 12-40 and 40-150 f2.8 Pro lenses plus 1.4TC and no other camera has a f2.8 kit with this much range in such a small package. For Sony I use the my Tamron 17-28 f2.8, 2 small 35 and 55mm S/Z Sonnar primes and 100-400 f4.5-5.6 GM (superb) lens plus 1.4TC. Not interested in a Sony 24-70 or 70-200 f2.8 as they are too large, heavy and expensive, nor do I see needing the Tamron 28-75 f2.8 zoom, as I would rather have the smaller primes for the mid range. Convenience goes to Olympus, but reach goes to Sony and the 2 AF-C systems are pretty close, just a slight advantage to the Sony A7RIII/A7III. I am debating on whether or not to add the new Oly 100-400, as with the 42MP Sony A7RIII, I can get a 18MP crop file that allows the Sony 100-400 with TC to go out to 840mm at f/8. Here the 42MP of the Sony has some advantages if you crop, plus I like the higher MP files better for natural landscapes with lots of detail.
For overseas travel it is the Olympus kit all the way. I can bring both the small primes and the 12-40 and 40-150 f2.8 Pro zooms, plus a Laowa 7.5mm f2 Ultra wide angle, that covers everything and fits into a Mindshift sling bag that weighs about 6kg. My wife and I spend a lot of time in Thailand (she's Thai) and the weather-sealing of my Olympus gear comes in very handy, as it can rain every day. Been using M4/3 for my Thailand trips since 2012 and am more than happy with the IQ results.
Edit: Typical 4 prime Portrait kit for each system below. As mentioned, not that big of a difference in size or weight. Nocticron 42.5 f1.2 on far left, Zeiss Batis 85mm f1.8 on far right are the most used for portraits on either system. Olympus 75mm f1.8 back left, Zeiss Batis 135mm f2.8 back right. Both Batis lenses have OIS, which is very helpful and work with the Sony IBIS. The Nociticron also has OIS, but does not work with Olympus bodies unfortunately. The middle lenses are a Sigma 30mm f1.4 left and Sonnar 55mm f1.8 at right, and each camera has a very small 35 (34 eq) lens mounted. The hoods are removed for front row lenses and yes, I have blacked out each camera's bright white name with a sharpie
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