I heard a talk from a D.O. cardiologist last night, somebody asked a similar question.
He always wanted to do cards. He said when he looked at IM residencies, he didn't necessarily look at the size of the hospital etc. but the size of the fellowship itself in relation to the number of Internal Medicine Residents they have. Now as a general rule, of course a massive hospital will have a larger number of residents and fellows. But among the medium to large sized hospitals it may vary among them. He went to a program that had about 12 Card fellowship spots each year (which is a high number) He said, that most programs fill about half of their fellowship spots internally, so that meant there were about 6 spots, and he felt his chances were good. He also said at that point nobody cared (even back then) if you were a D.O. or M.D. What they cared about, was getting to know you blah blah blah during your IM residency. So what he was saying is your best bet is probably a fellowship at your IM residency, where you can prove that you are worthy. If you are at a small hospital with 25ish IM docs, competing for 1 or two fellowship spots, not good
Pretty obvious, but something to really pay attention too if you want a competitive fellowship.