Hard decision, I have only ridden the XR250 and I'm fairly sure it will have softer suspension and seat and also a lower more upright seating position/centre of gravity.
If it is mainly tight rough offroad stuff I reckon thats the way I would go (if I had a bad back), they have a reputation of being a comfortable easy to handle bike, but if you have long fast stretches of road/trail you will be struggling to keep up with the DR's .
Tell you what why don't you pay for me to come along too, we will get one of each and you can ride whatever one takes your fancy at any time

Perhaps a better place to ask this question would be a motorbike forum such as this
www.dirtbikeworld.net/forum/index.php or many others that are out there.
My current bike is a 2009 Yamaha WR250R, fuel injection, electric start, etc.
For a little 250 it is amazing how smooth it is at 110-120 k's..... but the seat is way harder than a 1995 XR.