Hey,
Hard to say. Horsepower, maybe a couple more. The big difference is in the throttle response, and quicker reving, which gives the impression of much more HP. Regardless, you should be able to leave a stock XR250 in the dust. The '96+ models have better frames, and suspension I believe. They should handle a bit better. Powerwise, I'm not sure, the newer 250's might have more power. But the restrictive stock setup, on any year XR, really kills throttle responsiveness.
Are you just trying to justify your older XR or are you thinking about getting rid of your '91 for a newer model that somebody told you was better? Either way, stick with your '91 if it still runs good etc. If you want a newer bike, look at something like the XR400, DRZ400 etc. if you want more power. Good luck.