I had the xterra 50 and I really loved that thing...It had good depth and I noticed I did not dig as much iron stuff. The xterra 50 has a manual ground balance..............I think between the older 50 and the new 505 is the older 50 only could use the medium and high frequency coils. The newer 505 can take all 3 frequency coils just like the older xterra 70 did..........I am not sure about about the difference between the 505 and 705..........Since the newer xterras have hit the market the used xterra 50 and 70 prices have dropped considerably........In my honest opinion if you choose the older xterra 50 it will service you well as the older 70 would too. I do not know what kind of hunting you plan on doing but if you was ever going to do some prospecting then go for the 70 or 705. If you are not then just get the 50 or 505. I have read on this site that some peoples experiences with their xterra 50 said it would not pick up a coin deeper than 4 inches............Those same people either had a faulty machine, was at a place where no one lost anything or just did not know how to use a metal detector. Let us know which one you go with, Tim