I've had this conversation with myself about 100 times in the last six months , do I replace my CRF250 Rally with the 300 version, in the end i've decided to keep the 250 .
There are two main reasons for this ; the first is build quality. I have two mates who have bought 300's and they've both been disapointed with the quality, whereas my 250 is 5 years old now and has been ridden in all sorts of shit and muck and is looking better than their 6-9 month old bikes.
The second reason is that hardly any of the parts are interchangable between bikes which means I'd have to spend a lot to make a 300 trail worthy as I couldn't just use parts off the 250.
The 250 is by no means the fastest bike in the world but it puts the power down where you need it on a trail so I can live with the slower progress on the road.