I've got both populating my coat pockets.....
At the time I was considering the DIT book, I was told by somone at DIT that it replaced the red medicine book but was better as it had treatment protocols with doses, warmed your toast, spread the butter and actually poured your effin' coffee in the AM......well, I'll take BS for $400, Alex.
The DIT book is good - but limited in informational content. It contains Tx's for the most common complaints you'll see and based on what's in it, it looks like Dr. J's notebook for when he went through his residency.
But - if you're just starting out and don't have all the fun and games int. medicine stuff down yet off the top of your head, I'd keep the red medicine book. I can't tell you the number of times my attending would say something and I'd want to get a brief read on it only to not be able to find it in the DIT book. Now, if you've got access to UpToDate, that might take the place of the red book.
Personally, I'm looking at buying The Resident's Guide to Ambulatory Care and having access to the Merck Manual on my iPhone. What I've realistically seen on rounds is a heavily annotated red book and UpToDate for the latest,greatest stuff......
But I still carry the DIT book also.....