Not sure what kind of riding you're planning on doing, but there is a reason that 90% of the "125" riders at the local tracks are on 250F's (and 99% in the pro ranks). Chances are you will be faster on the 250F for all of the reasons you have already mentioned (easier to hit the inside line and make a jump, easier to stay in the power, easier to hook up/get traction, more consistent/dependable power than a 2-stroke, etc.). I rode 125's for 4 years, and once I rode a 250F (2002), I jumped ship immediately and will never go back to a 125. I will still do a few laps on a 125 when given the opportunity, just for fun, but I would never race one again.
And when it comes to maintenance, just check your valve clearances, and keep them in spec, change your oil, clean your filter, etc. and you should be in good shape.