Your talking about moving from a over-weight, under-powered trail bike to a motocrosser... doesn't matter if it's a 125 or 250 2-stroke, or 250-450 4-stroke; they are all going to feel like more than you can handle.
This is where that gray matter between your ears comes into play. Until you feel comfortable with whatever bike you end-up with, just take it easy with your right hand and don't ride tracks or places that will get you in over your head. Youtube is full of fail videos starring noobs taking dirt naps. Don't be one of those.
If you try to buy a bike for every level of comfort on your way up the experience chain, you'll be buying a new bike every month.