Seegy,
As Ralph stated, both are excellent detectors and leaders in that under 700.00 price range.
I've used both of them and they are competitive in Depth, with the edge going to the M6 in terms of lower conductive targets. The X-Terra 50 @ 7.5 kHz will be a bit stronger on the silver.
As Ralph indicated, both are Great Choices, each with their strong points regarding features; The Xterra 50 offers notching which allows you to ignore given ranges of targets. The Drawback of notching is you could notch OUT a surface or shallow target and MASK a deeper notched IN target like a silver coin.
The M6 offers Seven TONE ID v Four of the Xterra 50.
The M6's state of the art ground tracking gives it a big edge in terms of neutralizing ground noise. The Xterra 50's manual ground balancing is easy to engage but will not track when the mineral changes, so an occasional ground balance check is recommended in non-homogenous ground.
Both have a very good balance and feel. The Xterra has a lighter feel to me.
The Xterra also has a 18.5 kHz loop which definitely increases the signal on lower conductors, better than the M6, but the reaction to smaller ferrous is increased as in the Tesoro Tejon and Troy X3/5 series which operate at 17+ kHz.
A wider range of coil configurations is available for the M6.
Both are excellent choices at under $700.00.