Both Cardiology and GI have evolved into the most competitive IM subspecialty fellowships. There are approx. 650-700 Cards spots available in the US, and something like 250-300 GI spots. Because of the sheer need of Cardiologists by the marketplace, there is talk of increasing the number of spots of Cards fellowships. GI seems to be holding the line where they are. I think part of this thinking is related to the scare that GI had a few years ago, when the word on the street was that there were too many GIs out there and not enough work, so the # of people applying dropped like crazy and GI programs couldn't fill. Now that the demand for GI docs is out of control, the same fellowship programs are living high, taking their pick. If they're smart, they'll play the game like ENT or GU, keeping the # of spots small and stable so that there will always be a strong demand for the field, jobs aplenty, and the caliber of applicants high.
But I digress. Onto Cards. Cardiology is very competitive, probably still slightly more than GI for right now, but there are more spots so I think it evens out a bit. And while there are greater numbers of people applying for Cards, you can pretty much separate out the IMG appplicants from US applicants (except for the truly stellar IMGs), and then separate out socially-challenged US applicants from normal ones, and then separate out those US applicants from top IM programs from average ones. So the numbers do reduce themselves. Having said that, there are some facts/tips to keep in mind to make sure you've done all you can to enhance your chances of matching in Cards. I cannot stress enough to you the importance of going to the best name/reputation IM program you can get into. As Klebsiella touched on, what matters to fellowship programs is what you did just before applying to them. Coming out of a highly regarded IM program puts in you in a higher echelon of consideration, in that they (fellowhip PDs) already know you had the numbers to go to a reputed IM program. Everything I'm going to say now relates to the reputation of the IM program. As a general rule, the highly regarded IM programs will usually have highly regarded Cards programs, which means they will have highly regarded Cardiologists on faculty. These are the faculty you want writing your letters because they are buddies with and trained with all the other Cardiologists at fellowship programs. The world of academic cards is small, and all these faculty know each other pretty well, so a letter from a buddy is immeasurably more valuable than one from a stranger.
As a general rule, I think you do have your best chance at fellowship where you train in IM, for obvious reasons. I think notable exceptions would be places like the Cleveland Clinic, where the repuation of Cardiology training far outshines the general medicine program. That's why these people who think going there for IM with the intention of gunning for fellowship are in for a rude awakening. CCF takes their pick, and except for may one or two a year from their own IM program, they pick from top IM programs exclusively.
It sounds like you have your heart set on metro NYC programs. That's fine and good, and definitely focus your efforts on getting to know faculty and doing research with them with the object of staying in NYC. From my residency applying days, I seem to recall that Cornell, Columbia, NYU and Mt. Sinai were uniformly considered the top IM programs in the City.
But, if you want to do cards more than anything, you need to look past Eastern Pennsylvania and apply far and wide across the US, both for IM and subsequently for fellowship. I believe that if you left NYC and went to a top IM program away from NYC, then appying to fellowship programs in NY would be more advantageous than staying at an middle-rep program in NYC just to stay in NYC.
So, yeah, for IM, academic top-name places are your best bet. For fellowship, I honestly don't think it matters unless you're going into academia. My goal is the private world, and while I'd like to get into a top Cards programs, honestly my more important focus is to go to a volume program where they do tons of procedures and see lots of patients so that proficiency is not an issue. Along those lines, private places like Cedars-Sinai or Cleveland Clinic are places to look at because they have a lot of private-insurance and big $$ patients who can afford all those procedures.
With regards to $$, I really wouldn't worry. While deferring loans isn't a great way to start "real" life, you will have moonlighting opps in residency that will help pay off some bills. As for life after fellowship, that's even less worrisome. Cardiologists, even ones who do no intervention, do better than about 90% of other medical or surgical subspecialists in terms of earnings. So $$ should not be a worry.
Many Cards fellows I know moonlight. As discussed previously, fellowships vary greatly in terms of how hard fellows work, the amount of night call, etc. You just play it by ear, and moonlight more during lighter months (ECHO or a reseach month) than on your CCU months.
Oh, one thing about research. IF you've been working on something from med school days and can build on that during residency, that's cool -- the basic work is done, you're just looking at the data to make different arguements. I think if you're hoping to start some kind of a project in residency, it might be wise to start working on something from scratch with a faculty member at the end of your intern year or beginning of your second year. You're just not going to have the time or energy to do something worthwhile/meet deadlines as a intern, so all you'll end up doing is looking irresponsible to the faculty you're working with and delaying the research more. So only make the time when you know you can, which will be after intern year when your schedule frees up a bit. Just an opinion though.
Hope this helped a little.