Other than when hunting the more mineralized sites where the X-Terra 50 works well and the X-Terra 30 doesn't due to bad ground and the '30s' preset GB, they have been very, very close in perceived sensitivity.
Perceived, that is, by way of their detection depths and target responses. With both in the more sensitive (to outside RF interference) All Metal mode there have been times I can run the X-Terra 30 at the maximum '10' setting and it is quiet and stable. The X-Terra 50, however, might have to be reduced to '19' or '18' or '17' to regain the same level of audio stability.
One might think that this means the X-Terra 50 has a titch more sensitivity on the upper-end, but so far I haven't really noticed any noteworthy difference in depth of detection. This is with the X-Terra 50's GB set about the same as the X-Terra 30's.
I would still like to think, however, that there will come a time when IO will work a site and realize a little 'edge' in favor of the X-Terra 50. In general, however, their performance is darn close and I even tried swapping their coils to see if there would have been any difference attributed to the coil's dynamics.
I hope you have detectable weather when you get one in your hands.
Monte