I was a long-time 125 rider, tried the 4-stoke thing briefly, but they just aren't any fun for me. I'll give you what I've learned, after a long history on the 2-stroke. Keep in mind, I'm a woods rider now, and don't keep the bike as "pinned all the time" like an MXer would.
I never really followed the owners manual to a T...(rings every few races). I did rings religously every 30hrs. and then a top-end at 60hrs. Wear was always good and predictable at these intervals, and not too costly. (never have suffered a top-end failure due to excessive wear) Rings and base gasket...20-30 bucks. Complete top-end...80-100. And I always split cases to take a look at lower-end every-other year..(every 2 years). I've heard the YZ125 is a long-lasting bike and if it was me in your shoes, I'd go 125. Use a good synthetic 2-stroke oil at a safe ratio, keep air filter clean, change tranny oil about every 5hrs., and ALWAYS let bike warm-up...(cold seizures are easy to do on a 125). Find a good jetting combo acording to temperature, write them down, and get used to re-jetting to fit the weather. Smoke 'em on the downhills!