I did rotations in both GI and Urology. Both are great fields.
I found GI to be a more interesting in the medical aspect than urology. Although many patients have peptic ulcer disease or want colon cancer screening, you also see everything from patients with Inflammatory Bowel disease who may have symptoms like skin disease or arthritis to the young patient with Irritable Bowel Syndrome who needs some reassurance. Research in GI is on the cutting edge right now especially with treatment of hepatitis. Also with GI, you will be very knowledgable about medicine so you're not left confused when someone asks you about their chest pain or what the side effects of some medications they're on. There are also a number of procedures you can do with the scope. At the beginning, many GI's might take an hour or something to do a scope, but after several years of training, you can do it in 10 min and bank whatever it pays for ($1000?). You can also select your practice to involve no patient care and just scoping all day from referrals, the result is a huge bank account.
I think total training in GI is 5 yrs. It seems like a tougher residency/fellowship at my institution than Urology. THe easiest way to get in is to do well in med-school and get into a higher ranked internal medicine program. If you don't do well in medical school and go into internal medicine, you can renew yourself by doing well in the residency.
Urology has more interesting procedures than GI. Alot of it involves cystoscopy which is scoping through the urethra into the bladder, but you also get to do things like open the abdomen for a radical nephrectomy. I also saw lasering kidney stones in the ureter which is almost like playing Asteroids, or sitting in for an Extracorporeal Shock Wave Lithotripsy where the urologist just stands there for 45min watching the technicians doing the work while the urologist goes home with a nice check. But the medicine side of it seems pretty boring, even for me who finds penis interesting. The erectile dysfunction patients are interesting, but this is not the bulk of the specialty. And unfortunately many urologists seem clueless about general medical issues, but this goes for several surgical specialties. I think the hottest research right now in urology is in the management of prostate cancer, right now there is much controversy on this subject.
I think total training is 6 yrs. To get in, you need to do well in medical school.
Okay, I could right a novel