Another one - What bike should i get?

Hey everyone,

First of all a small introduction: I am 18 years old, 150lbs, and a solid local B rider.
I am currently riding a 2015 kx250f, and i really like the bike in terms of riding and racing, but what i dont like is the maintenance and reliability of the 250f.
I ride about 100 hours a year, and the maintenance of the 250f is just too expensive and too stressful to keep up with.

So i wanted to try out something new, and am currently deciding between a new 2016 kx450f, or a used 250 2 stroke (yami or ktm) and hop it up a bit.
My initial thought was that the 450 would require less maintenance than the 250f, and the 250 2 stroke would require as much maintenance but would be a lot less expensive.

But, of course, i also want to improve my riding and get faster, and i fear that the 450 might not be the right bike for that.
So, which bike should i get? 450, 250 2 stroke or should i really just stay on the 250f to improve my riding and somehow try to keep up with all the maintenance?

I know, lots of questions. hoping to get some helpful answers