It was a close call when buying my 172 whether I should go for a CTR, there is still a little voice in the back of my head saying "why didnt I get the CTR" Well the CTR is slightly quicker, and has a higher top end but in all the magazines I am yet to see one beat a 172 on a track or on a 1/4 mile. 172 has lots more kit as standard, and is a smaller lighter car, does need 17s though. CTR is a different sized car more comparable to the RS or R32.
172 is the fastest car in its catagory (Yaris T-Sport, VTS, Polo GTi, Cooper S). Unlike the CTR which is now out paced by several (147 GTA, Golf R32, Focus RS.)
They are both great cars and I will probably end up owning either a CTR or R32 in the future.