I do not believe there will be much difference between the various aftermarket choices and the stock silencer. The XR250 exhaust is not the bottleneck in the HP department. The last dyno tests I viewed show less than 1HP difference between the stock pipe and the others. Once some mods are done such as a big bore kit, that will change.
The stock headpipes and muffler are fine, it is the tip that is the problem on the stock muffler.
On my friends XR250 we: 1. Removed the airbox lid.(FREE) 2. Installed a UNI filter with a high flow cage. ($30) 3. Ground the welds on the headpipes. (FREE) 4. Took out the stock tip and replaced with a baja designs tip. (~$90)
After all of those changes the bike was substantially more powerful than it was with just the airbox removed, and yes it was jetted correctly before we did the mods.
You can do all of that for under $160 including jets so I would recommend trying that instead of wasting money on an aftermarket exhaust.