Choosing what type of subspeciality you want to do is not easy. But it essentially boils down to some extent into personality and what type of people you mesh with best, how much you like/want to do procedures, what field of study you find most interesting, what role for a patient you want to play (primary provider, consult, proceduralist, etc), and what type of work will give you the most fulfillment at the end of each day.
I would suggest keeping an open mind as some fields lend themselves to certain people very easily and for a lot of things you won't get a lot of exposure to in residency. There are lots of fields with lots of different ways to practice medicine. And be sure not to forget about general medicine. I know this has been written before to a degree but below are my quick thoughts.
1. cardiology - all heart stuff, procedure heavy, they like to "do" things, high consult load, high patient load, long hours, rely a lot on imaging and less on the clinical aspects we are taught, TONS of guidelines and studies you need to know/memorize; can get a little repetitive in the basic stuff but the subspecialized cardiology stuff like CCU, advanced heart failure, interventional is all really cool and really interesting
2. GI - GI stuff, see a LOT of psych patients, procedure heavy, not has high consult load but lots of GI bleeders, most consults are for procedures, rely a lot on imaging; do this if you love procedures
3. Pulm/cc - very broad field and see very sick pts usually, divided into pulm and ccm; pulm has bronchs (has fellowship for interventional too) and consults are very intellectual, ccm - critical care and all it has to offer (very broad and your work when you finish training will depend on what type of hospital you work for --- micu, sicu, ccu, neuro, etc will vary); pulm may also have to consult on vent pts in other ccus
4. ID - very intellectual, no procedures, often is the lead consultant for many patients seen in my opinion, your treatment actually can fix the underlying problem and many patients will have defined endpoints with you, you really do need to like HIV though because a huge number of patients you see will have it
5. Heme/onc - now sometimes being separated out, few procedures outside of BM bx, need to like path because that is big in heme; onc is very multidisciplinary driven nowadays but the medical oncologist is usually in charge of the patient, onc has lots of guidelines; treatment can cure devastating diseases; play a huge role in pt/family lives; is sort of the "it" doc nowadays in popular culture
6. Renal - some procedures; have to really like CKD and managing dialysis patients, in my opinion clinic is usually pretty easy and I personally like it the best (quick visits, pts like you, other docs always need you), a lot of inpatient work too, see a lot of really sick patients and can make a good bit of them better; and in my opinion renal really does do some great things for pts with HD because it can so dramatically increase their life span more than lots of things we do
7. Rheum - some procedures; almost all outpatient; can be very intellectual as you get pts other doctors can't figure out; pts are usually younger (expect for RA which is your mainstay), mostly if not all autoimmune stuff
8. Endo - not sure if they do any procedures; almost all outpatient; diseases usually aren't that complicated and it isn't hard necessarily to figure out the next step; you do get to see some interesting stuff in terms of finding diseases other people tend to not think too much about
9. General medicine - I think we all know this; can be outpt or inpt; pretty huge variety in type of job you want to have (nights, days, weekends, the number of days you work, academics, private practice, etc), pretty broad, always have someone to go ask for help in terms of a consult if you are unsure of something
10. Other -- there are other types of fellowships/career paths you can do as well outside of the traditional pathways mentioned above and these are pretty varied... palliative care, geriatrics, genetics, ethics, sports medicine, research fellowships, epidemiology, pubic health, politics (many politicians were doctors), etc etc. You just gotta do some research and see what's out there. And do some soul searching...
hope that helps for future readers