the main difference is having to do an internal medicine residency to become a nephrologist... yuck
nephro - pts are diabetics, hypertensives and people with weird kidney problems. plenty of time tweaking dialysis machines too. one nephrologist told me the joke "why do they put nails in the lids of coffins? its to keep the nephrologists out." surgery is minimal unless you put your own dialysis ports into your patients. its medicine, pure and simple, only you're a bit more focused. 3yr+3yr program (i think) - no lack of a job market.
uro - more surgically oriented; not focused purely on kidneys. have to spend some time doing general surgery during your training. have to know some nephro but nothing like nephrologists. don't deal with weird stuff like lupus, fsgs or glomerulonephropathies. as far as kindeys go you can image them, scope em, clean em out, take them out - but don't really manage them over the course of someone's life. if you are afraid of doing rectal exams or talking about people's special parts its not a job for you. 5-6 yr program - no lack of a job market.
remember, this is just one persons opinion. if you are a student then do a rotation in both fields, its worthwhile.
