Here's what I found. A lot of the studies seem to be about women; and I wonder if that will make a difference. However, it seems that a lot of sources say that Animal protein is actually harder on the kidneys. This makes sense to me. Anyways, feel free to check these out if you like.
[/I]http://www.news.harvard.edu/gazette/2003/03.13/09-kidney.html
The research team analyzed kidney function by measuring the estimated glomerular filtration rate (GFR) - an indication of how effectively the kidney is filtering the blood in order to remove waste from the body.The data suggested that the type of protein consumption was also an important variable. A change in GFR was only significant among women consuming large amounts of nondairy, animal proteins.
http://abc.net.au/science/news/health/HealthRepublish_810492.htm
The research by the Harvard School of Public Health in Boston found that high total protein intake, particularly animal protein, may accelerate loss of kidney function in women who had mild chronic kidney disease.
However, animal and plant proteins differ in their effect on your kidneys. Consuming soy protein is associated with kidney changes comparable to those resulting from a low-protein diet. This raises the possibility that people with kidney disease may be able to liberalize their protein intake by using soy protein. (Do not change any diet prescribed for kidney failure unless directed by your doctor and registered dietitian).
http://www.solaeliving.com/healthbenefits/otherhealthbenefitsofsoyprotein.jsp
Research has also shown that consuming soy protein instead of animal protein decreases levels of harmful fats in your blood.