Doghouse: The main differences between the two models can be seen on the new features that the X-Terra 70 offers:
New: More Coil Frequency options.
New: Prospecting Mode.
New: Salt Water beach Mode.
New: GB options: Manual/Auto/Track .
New: Threshold Tone.
New: Discrimination Patterns (3+ All Metal).
New: Sensitivity Adjustments.
New: Noise Cancel Adjustment (5 + Auto)
The main differences will be the Prospecting Mode and the Ground Tracking Modes. I think that otherwise, the two detectors are fairly similar in many respects. That's also a good thing if you are already familiar with your X-Terra 50 because the 70 will not be too much of a mystery to you the first time you pick it up.
Most users won't have need for the Prospecting mode for example... but then again, many users find Prospecting Mode to be more sensitive. I've experienced this myself. I'll sometimes use Prospecting Mode for coins etc and then switch to Coin & Relic Mode to try to ID the target before it is dug. Sometimes when I switch to Coin & Relic, the detector can't locate the target found in Prospecting Mode.
Cheers,
Marco