2 stroke 125cc vs 4 stroke 250cc dirtbike?

If you don't intend to race and money is an issue I think you should go for the 2 stroke 125cc. They have plenty of power but are not what I would suggest for someone who wants to take racing seriously simply due to the fact that more times than not they have to line up against the 4 strokes and then the 4 strokes usually have the advantage. But not racing you won't care about that. As far as noise goes the 4 stroke is a louder deeper noise that tends to carry further. I know I worry about the noise from our own track as my son used to have 2 strokes and the difference was very noticable when they moved to the 4 strokes. If you are coming off the crf150f you will find a 2 stroke 125cc more than enough of a handful as they are a lot different to ride than the bike you have. Just be aware that if looking at the cr125cc honda stopped producing them in 2007 and they did no further development on the bike from around 2005. Ktm will cost you more and the parts are usually more expensive.