Depends on several factors.
An example of circumstances where the X70 may have greater depth.
1. Rapidly changing ground where the GB Tracking provides a better GB.
2. Saltwater beach where the beach GB provides a better GB.
3. Stability Mode where reported target confidence provides more target information, especially in bad ground.
4. Capability to use a 3kHz coil which should provide better depth in bad ground.
5. If notching is used, the higher resolution allows more adjacent non-notched targets to still be ID'd.
6. For those that prefer a threshold based hunting style they may use it to their advantage on the X70.
7. The Prospecting mode IS deeper, and may even correctly ID targets in Full Iron mask that most VLF's including the X50 will call iron under certain ground conditions. This may be a bit of a stretch, but I think of the Prospecting mode on the X70 as almost like having a dual technology detector, a VLF & PI in one machine.
These are just a few things that come to mind.
HH
BarnacleBill