Keeping the IDX Pro with MBMod makes sense to me, as it gives you a Very Quick 6.59 kHz Look into the ground. Its NO Prizm, but that's not saying its good. It's older technology, but so is my Compass Coin Magnum and White's Supreme V, and both are still fun to play with and will find things missed by others.
The M6 (White's Latest Model sharing the basic MXT Chip but with reworked software) or MXT would give you 14.7 kHz, so more sensitive on Low Conductors.
I'm hearing a lot of good feed back both from my Customers and others on the Xterra-70 which I haven't had a chance to use yet (have used the Xterra-50 and also love it.
One of my customers, Howard H, who has BOTH the Xterra 50 and 70, likes the Prospecting Mode of the 70, as it is similar in its response, like a Sovereign GT or SD Series Detector. The 70 LIKE the MXT/M6 has Ground Tracking, not so critical for coin hunting, but is a positive factor during changing ground conditions while gold nugget hunting or at a salt water beach. The Xterra although limited in its coil sizes, the three 9" concentric coils change the frequency from the middle or standard 7.5 kHz to a Lower 3 kHz (High Conductive Silver) to 18.5 kHz (Low Conductive Gold).
However, higher frequencies above the old Garrett Ground Hog Standard of 15 kHz means small iron is going to react more and rejecting it cleanly becomes a bit more difficult. I prefer the 12 to 15 kHz levels for BEST Iron V Gold Signals.
With the MXT, you can hunt in three discrete modes (Coin&Jewelry, Relic, prospecting) and enjoy full resolution VDI. The M6 lacks the Prospecting and Relic Mode, but seems to have a bit smoother operation in the Coin and Jewelry mode over the MXT, along with a new Seven Tone ID System you can turn on, especially when running no Disc in the Motion Mode, works great on the beach.
Start saving and banking that Clad/silver your finding with your Mr Bill Modified Classic IDX PRO.... On a regular basis, not to mention any rings, you should have more than enough coinage to assist in your purchase by any of the aforementioned highly regarded models.