The 505 is a threshold based machine that is lighter, capable of running in one of three frequencies depending on coil selection, has an adjustable armrest, has choice of 1,2,3,4,or 19 target ID tones, 19 target ID segments, has adjustable volume (headphone and speaker adjustments are independent), has Noise Cancel (a feature that is particularly useful when hunting near another Xterra), and a year longer warranty that is transferable.
And FWIW, there are a total of 12 approved coils available. Five HF coils (18.75kHz), four MF coils (7.5kHz), and three LF coils (3kHz).
The AT Pro comes standard with a DD coil, has an auto GB option, Iron Audio in Pro Mode, fixed volume, three tones, 12 target ID segments, single freqency (15kHz), no threshold tone, non-adjustable armrest, is waterproof to 10ft, and costs about $50 more than the 505.