I own both a YZ250 and YZ250F. They are both older models, but feel like I can give some input regarding what it takes to keep them running and as well as riding them back to back. I dont race MX, just a couple C Senior Scrambles a year and the rest tough single track. Put 150-200 hrs a year on the YZ250, my primary bike and do all the maintenance on both bikes.
The two stroke once sorted for your riding style will cost significantly less and take less time to maintain. Its also cheaper to change the state of tune to match your riding style. I generally go no more than 100 hrs on my top ends of either bike, checking at 25, 50, 75. If my compression or leakdown figures move before 100hrs, I tear down sooner. The two stroke crank seems to last longer than 250F fourstrokes (I dont know why). I've got over 300 on the current YZ250 crank and will be changing out the next top end. I've seen some 250F heads shot in less than 100 hrs.
My younger riding buddies who are still racing a lot of MX and at a higher level than my pace are changing out their pistons at anywhere from 25-50 hours of either flavor. No race bike is going to give you ride and forget about maintenance regardless of flavor even if trail ridden mostly. Thats what TTR is for.
The YZ250 is more fun for me to ride offroad in tight technical terrain with its low rpm instant punch and lack of flame out, thats why it gets the most hours each year. The 250F is easier ride for longer periods, and frankly just as fast or faster in flowing terrain for me. Just takes less to stay on top of it when going fast. Pretty much the same as the mag articles or any of my riding buddies who own both will say.
If you are concerned about maintenance, I'd really caution you about buying a used 250F unless its a very low hour bike and you know its history even if less than a year old. Its all about hours and maintenance. Any bike I have ever bought used was immediately torn down and given a fresh top end to ensure I know what I have. Once a 250F blows, they are very very expensive to rebuild. Hence a used YZ250 being a little less risk to buy used.
Just my experience, others may vary. Both great bikes!