My experience is as follows:
1. Both Medstudy and MKSAP are good sources of reading for IM. They are more clinically focused than Harrisons (which has large amounts of text focused on pathophys, which is not really tested on the ABIM boards).
2. The questions in MKSAP are much better. In MKSAP 14, esp when using the CD-ROM, the questions tell you what proportion of people get each question right and wrong, so you get some sense of how you are doing. There is no way to convert a MKSAP question score to an ABIM score.
3. The text of MedStudy is more outline format, and to the point. In general, for my residents who I feel are in danger of doing badly on the ABIM boards, I strongly encourage the use of MedStudy. It clearly deliniates the "key points" to focus on. The MKSAP text has come a long way (it was quite bad when I used it to study), but I feel it is directed more towards an "advanced" user.
So, short answer: TEXT from MedStudy, QUESTIONS from MKSAP.
There are other good sources out there -- Mayo review, etc.
I can't stress enough the importance of reading regularly, about the patients for whom you are caring.