I sell both brands. I have noticed in the last year or so that the Toyo's stopped lasting as long as they used to. But my Nitto rep was the one that gave me the info. From what I've seen though, the weight is the same on 90% of them.
My Toyos were purchased in spring 2009 and my Nittos were purchased in fall 2011 if that makes any difference. I can tell you though, from my experience, my Nittos did significantly better on wet pavement than my Toyos.