Well the Omega has an auto ground-grab feature as well as manual ground balancing whereas the 505 only has manual. I opted for an Xterra as I'm in Australia and it was more suited to the terrain plus the Omega has US coin indicators whereas the Xterra just uses the numbers. Dimes, quarters and nickels have no meaning for me. When I was looking for my detector my research led me to believe depth and sensitivity seemed about equal although the Omega seemed more chattery on dodgy ground.
You might be better off saving a bit longer and getting the 705 if you decide to go with the Xterra.
Edit: If you don't need a visual display take a look at the Tesoro Cibola