I have read here and there that the PSR-500 is more sensitive than the Pro-651 and that it is good on mil air whereas the 651 is weak on mil air. I have a 651 and am reasonably satisfied and looking at a second hand 500. I will add the 500 simply for a second radio for searches but if it is infact a stronger receiver all the better. I'm interested in hearing from anyone who has done side by side comparisons.