Just to explain, the ATP is a 15kHz machine. The four frequencies are only small deviations from the main operating frequency. For example, your choices might be something like 14.95kHz, 15kHz, 15.05kHz & 15.10kHz. Their intended purpose is to reduce interference from other machines and/or EMI/harmonics that can wreak havoc, and aren't enough different from each other to effect any significant change in target response.
By comparison, the 505 can run 3kHz, 7.5kHz, or 18.75kHz depending on the coil selected, which has a dramatic effect on target response. It also has five minor frequency changes available for all three frequencies similar to the four that the ATP has.
If you like the one size fits all kind of detector, buy an ATP. If you want to be able to tailor your setup for specific use, then the 505 is a more than competitive option on land.