I have an ATP and a 705. I realize you are asking about the 505 but I will still add my comments on these two.
GREAT MACHINES! I love both and it's often difficult to decide which to use.
The 705 is slightly lighter in weight. Not much but if your friend has any physical issues it can be important.
The 705 seems to find more but the ATP I believe pinpoints better. I also find the ATP gives false signals when the coil hits tall stalks of grass and the 705 does not.
And the 705 has a much larger learning curve than the ATP and the ATP can be submerged and the 705 can not.
Flip a coin.