Well i just went from a 350 FC 2019 with Factory KYB fork and shock to a 250 YZF 2021 (GET ECU, full fmf pipe). You can find both of my bike on "bike checks" or in the bike build topics.
My 250 YZF has the suspension revalve to my weight (210 lbs) and man i tell you right now it is my favorite bike by far !
I didn't gel with the 350 (but many does so don't base your choice on one opinion). The power of the 350 is soft down low (which is fine) but then hit like rocket ship mid/up top. It's not a 250 and it's not a 450 in terms of power so after 40 hours on it, i never get confortable.
Then the new frame, for my speed (B rider) and on my tracks (hard packs all year long) the new frame is too rigid (compared to my 250 sxf 2017) and the handling is not like a 250, don't expect to felt light like a 250.
Now onto the new Yamaha, man that bike has really good low end (even for heavier rider, you can use 3rd in many corner if you want too) and the GET ECU gave me the top end power i was looking for.The handling is awesome, so stable, safe and yet remain light in the corner.
For a B rider weekend warrior the new 250 YZF is just perfect. But if you are a racer, A level, maybe the 350 will match your speed better