the 250r used square tubing the ex round the ex is taller to accomadate the motor. Also the weight on the ex is shifter rearward, this is supposed to eas sterring but I think it was to allow better wheelies.
The R frame was not very good mine lasted about 3 seasons but I was always welding on it. The after market frames are closer to the ex frame which also needs some gussiting for racing but on whole is much stronger than the R. Remember there is 12 year in tech and metalergy in the ex frame.

 

I thought the article said it was not based off the CRF450 motor but was something different that was easier to maintain with a higher reliability for the average rider. They did say it was supposed to have a lot of hp though. I guess we'll have to wait and see.

 

it said and I quote"expect it to use a 450cc liquid-cooled four-stroke power plant that is quite a bit different from the CRF450 motocrossers".