The urologic oncologist concentrates on cancer in the urinary tract. This doctor is familiar with removing cancerous kidneys and may also handle partial nephrectomies in which only the cancerous part of the kidney is removed. (A partial nephrectomy is a much more difficult and serious operation compared to removing a whole kidney). The urologist oncologist also knows about further treatment of kidney cancer including drug treatment.
The urologist treats all sorts of problems in the urinary tract. All urologists are trained as surgeons and have experience removing kidneys, but a regular urologist may not have much experience with removing cancerous kidneys. (You can always ask what their experience has been, how many cancerous kidneys or kidney tumors have they removed, what complications they have encountered, etc.) A regular urologist won't know much about kidney cancer, especially after it's removed.
For people who have kidney cancer and have a choice between a urologist and a urologic oncologist, I'd definitely choose the urologic oncologist.