Great question.
I will preface the answer with the following: I hated internal medicine my 3rd year. Every minute I was on the wards either actively admitting a patient or rounding (mental masturbation) with the team, I was miserable.
The thing that you will start to realize soon enough is that there isn't much House-like brain work involved in medicine anymore. Most of your management no matter what specialty you go into is gonna be guided by radiological or serologic diagnoses. There are, of course, odd entities that you will encounter now and then that require one to step back and see the whole picture and tie in physical findings with the lab tests to arrive at a plausible diagnosis. These are few and far between. Even then, most of the time the IM doc will be consulting other services (ID, pulm, endo) if an exotic or rare dz is suspected. As an IM doc, 90% (I really do mean 90%) of the time you are dealing with chronic illnesses: DM, CHF, HTN, COPD, Chronic Pain, Morbid Obesity.
Similarly, with urology, you can arrive at most diagnoses based on the scans alone. I say most because again there are times when you see a huge renal mass on CT and you have no idea whether it is cancer or not. Urologists probably do put more thought into their diagnosing compared to general surgery simply because urologic procedures and surgeries are more involved, from a technical stand-point. Moreover, there are several ways you can approach any given urologic problem. With renal masses you can freeze them, apply heat, take them out, or observe. With prostate ca. you can give pts radioactive seeds, perform robotic surgery, do an open surgery, cryo, or xrt. Management options are vast and depend largely on the patient. Even "small" stone procedures have multiple management options - shockwave vs. extraction vs. observation etc. etc. This is where the thinking usually comes in - in the managing of urologic issues rather than the actual diagnosing.
I am obviously biased but I would urge you to definitely look more into urology. Hope somewhere in the above rant I atleast attempted to answer your question.