as your turning 17 and on a cbt the max u can ride is up to a 125cc, u have the choice of reliability vs speed,
if you want minimal trouble and an avrage speed of say 75-80mph restricted then go for a 4T yzf-r125 (dont know about the de-restricted speed)
if you want common problems and regular maintance and some where around 85mph restriced(i think) and 100mph+ de-restricted go for a 2t rs125
i used to ride a rs125 but got sick of spending out time and money on engine parts and oils. not to mention the waiting for it to warm up before you can ride properly.the speed is great on it and its a good bike, but there is a lot of maintanance involved after a while and if you dont look after them. and now with the new ones 07 and upwards i belive that there harder to de-restrict to get full bhp as ther more complicated cdi's
so ive just gone and bought a yzf-r125,(still running it in) great little bike, as i dont want the hassel of warming the thing up and having to buy oil every week and wondering when its going to stop working.
if you dont care about the maintance side and just want the speed go for the 2t (rs125) so maybe a second hand one fully de-restricted and use the rest of the money for parts when u need it.
Or if you want a more reliable bike and not alot of hassle from it. go for the 4t (yzf-r125).