My work does a bunch of work with both bikes, but mainly the CRF150.
But anyway, I would go with the 150 hands down. They have more power, and a much bigger aftermarket if you wanted to go that route.
My cousin has an 05 CRF150 (probably 5'7" and 135 lbs) and it has plenty of powert to move him around and do wheelies and stuff.
Then my family has a TTR125, and it is definitely a smaller bike than the 150.
One of the BIGGEST downfalls of the TTR is that fact that the frame is not connected under the motor. The one we have has a cracked frame behind the motor from where the bike has been ripped apart from riding (if this makes much sense). I'm sure other companies sell parts too, but I know my work (
http://www.bbrmotorsports.com/Home/Home.aspx?s=true, the parts can be spendy, but it is made up for in the costumer support!) sells a 'frame cradle" which not only is a skid plate, but it attaches the two parts of the frame, which keeps the motor from being the only thing holding the frame together when subject to riders that weigh anything more than a small kid. Even if you are a kid, it is a good idea to save the frame.
And again, I'm going to refer to a part form my work, but we have the "rev box" for both bikes which replaces the stock box, and moves the factory rev limiter up a ways, and also changes the spark curve.
I have experience with the rev box of both bikes, and on the CRF150, it was the difference in be able to pull an actual wheelie with a 180lb rider, and not being able to. But the TTR does have more power with the box, but still doesn't have the power of the CRF.
Hope that helps, and if you have any other questions I'll do my best to answer. I'd go with the 150 though.