CMD&T is a great book to help learn some quick stuff about a disease or to help compare a disease to other differentials. However it doesnt' go into the basic science explination of the diseases like Harrison's does.
As a PBLer I use the CMD&T when we first open a case to learn about the disease and rule out other diseases. Then I go to Harrisons as specific stuff comes up for the case.
If your trying to decide what to buy -- In retrospect I wish I had bought the pocket Harrison's. I was given the big one and it's great but it's not very portable.
See I got CMDT as a gift from my cousin (who graduated from PCOM) when I started school, but everyone raves about Harrison's so much I wasn't sure if I needed to buy it, I hope I don't because I'm kinda poor white trash at the moment.
I wouldn't worry about it for now. The library has a copy of it and Cecils is the required 3rd year text. Even then I wouldnt worry about it, cause the pocket version is cheaper and most hospitals should have a copy you can borrow if you need something bigger.