My wife rides the TTR125L and my son now rides an XR100R. The XR70 would outrun both of them.
Power to weight ratio is the culprit, unless you've traded around bikes. The lighter rider (your son) is able to get more top speed out of the lowest horsepower bike (of the three) because his weight is less.
You lose 1 hp for every 7 lbs of weight at top speed.
About the choice between the XR and the TTR.
Regarding engine reliability. Both are bullet-proof. Both have been in production for the same period of time. Contrary to popular belief, the XR70 engine IS NOT the same as the old Trail 70. There are many differences.
Pick the bike which fits your daughter the best. Don't worry if she outgrows it quickly. It's the same as buying shoes, you don't purchase a pair of size 10's when the kid wears an 8. Most young riders do so much better on bikes that are the right size (or even too small). It's easier to learn, easier to pick up and start all by themselves, and generally more fun.
Don't worry about resale -- even if you purchase one of these bikes new, resale in a year or two won't be painful. Prices stay high, just check 'em out in your local Cycle Trader (if you can find any in there!).