Can an us tell the difference between an benign tumor or malignant on the kidney or is it to hard to tell? is mri the better route?

Can be tricky: U/S is good for simple cysts with no solid parts or wall irregularity (pictured). Beyond that it is a betting game. U/S has a much harder time characterizing solid masses. MRI and/or good contrast CT provide MUCH more info, but even then tissue diagnosis by imaging alone is sometimes tricky. Needle biopsy is back in vogue: http://www.jurology.com/article/S0022-5347(07)00779-3/pdf