Although both bikes are popular, not many experienced riders have ridden both, since they're considered "beginner" bikes in most circles.
Don't get angry because people don't choose to respond to your request. You won't catch many friends that way.
Anyway, I have ridden both, in stock and modified form.
Here's my take - the XR 200 is the better of the two if you're tall enough to ride it. The bike is designed for small adult riders. The stock horsepower is about 10 - 11. Modifications can make this a great trail bike, and there's lots of guys who chose this bike for tight, nasty mud riding.
Engine modifications can bring the hp of the 200 up to about 20, the same as an XR250. At this point, they're a very strong and fun bike. Unfortunately, the chassis isn't very good for aggressive fast riding (too flexy). Suspension components can be changed and the ride is better, but still not as good as a stock XR250.
The TTR125 is built for a smaller rider in mind. Although BBR components can be added to make the bike fun to trail ride for someone under 5'4", or for taller riders in motocross.
The engine is good, stock horsepower is about the same as a stock XR100 (8hp), just a little more lively. The engine can be easily modified (with a Powroll 150cc kit and cam) to put out a respectable 15 - 17 hp.
Now, how about giving up a little more information? What type of rider, what height, how much riding experience? Where will you be riding? What do you expect out of a bike?
A little more information from you will bring more information from the excellent group of people on this forum.