Do you mean Harrison's Manual of Medicine or the full-on Harrison's (the ten-pounder)? I'm assuming you're looking for something for third year medicine clerkship?
Both Harrison's and CMDT are excellent texts but they are very different and as such can't really be compared. Harrison's is the bible of internal medicine with long, detailed, advanced discussion on every disease out there. CMDT is more like a quick reference. It is still more complete than a med student reference book (e.g. High Yield Internal Medicine or Blueprints) but has nowhere near the indepth discussion of pathophysiology, etc. that you would find in Harrison's. I have a copy of Harrison's and I used it mostly for formal presentations about a topic that I would have to do during attending rounds or for topics that I really wanted an in depth discussion of (like diabetes). I didn't buy CMDT but there was always someone around who had it. It was useful for looking things up quickly while people were trying to decide on a treatment plan or differential diagnosis. To attempt to look something up quickly in Harrison's is much harder.
