Have not used the AT PRO or Xterra 505. But I have as my main Coin and Jewelry machine an M6. I also have 2 Xterra 70's which are also very good machines. The M6 is I think superior for coin/jewelry in parks and schools. It is fairly deep, having no problem getting coins at the 8 to 10 inch range in moderate soil with the stock coil. It has great ground tracking, really fast target response and hits hard on gold. Once you have found a couple of pieces of gold jewelry, you can usually tell by the sound that it is gold. It has a really large, easy to read display and is easy to use and really nice 7 tone audio. For relic hunting I use the 10.5 DD coil which gets a couple of extra inches in depth and I use the 6x10 DD for most of my coin and jewelery detecting. The only negative that I could say about it is that it tends to get a bit heavy after a few hours. The Xterra 70 is the big brother to the 505, as such I can say that the Xterra 70 with the 10.5" MF coil is super deep and great for relic hunting, that is the prime reason that I have mine. It gets at least 2 to 3 inches deeper then my M6 with a comparable 10.5 coil on the M6. It is lighter and has a equally good display, and I really like the 99 tones (actually 28). The down side to the Xterra in my opinion is that you have to go a lot slower then the M6, as the reset speed is a lot slower and the response time is also a bit slower. As your primary detector either would do well for you as both are solid well built machines with outstanding performance.