cessna 172
Sean
You will have no problems flying a 172 after the 150, The 172 is a much more stable aircraft, as it has a bit more weight and power. Infact your problems usually happen when you revert back to the 150, as you tend to forget that the 150 will not do all the things that the 172 will do, like climb in a hurry. Usually as you advance up through the types of aircraft, the response and power just get better and better .The only stumbling block is the unforgivingness of more advanced aircraft, make a mistake and you are liable to pay for it. 150s are very forgiving, thats why they use them for training. The 172, is forgiving too , try stalling one and youll find out that its takes some effort or total stupidity to get it to actually stall.
I also recommend that if you read the first 3 Trevor Thom books you will be well prepared for your exams. Enjoy learn and prosper