I have the 505 and thought as you did. In hind sight the 705's features are worth it.
First of all there are three models in the current xterra series 305, 505, 705
305 can handle 2 coil freqs 7.5khz and 18.75khz
505 and 705 can handle 3khz 7.5khz 18.75khz
all have manual ground balance
one or more search settings
pinpointing
all metal mode
what sets the 705 apart is the prospecting mode the beach mode sizing pinpointing mode automatic ground traking and ground balance offset
I would recommend going to minelabs web site and reading Randy Hortons Ebook understanding your Xterra
These are very good machines for the money