I've owned all three of the models you mentioned. Similarly, I have also had the new ID Edge and the X-Terra 30. You can find plenty of posts on here about my impressions (and finds) of the X-Terra series. As well, you can find a post on another forum of my impressions of the ID Exel and the ID Edge. Ironically, I sold the Fisher Exel and Edge for the two reasons you mentioned. Lack of depth and small (difficult to see when hunting) icons on the LCD. Those icons along the right hand side are virtually impossible to see unless you pull the display up in front of you. Depth is better on the Edge than the Exel, but still way short of the X-Terras. I also didn't like the fact that the user cannot make adjustments to the preset modes or modify the notching patterns.
I sent my first M6 back to the distributor as it would not allow me to run the sensitivity up past 60%. Naturally, that that had an adverse effect on depth. I just got in the replacement last week, and am hoping that it will show more stability. So far, I will say that it does not chatter as much as the first one. But I find that when I hunt in the multi-tone mode, I can't touch the coil on the ground or it sounds off. Changing coils does not help. The 7 tones might be a good idea for some. But as a coin hunter, it really don't make sense to include all copper, silver and clad coins into the same high tone. The X-Terra offers different tones for dimes and quarters, for example. I like the aspect that there are many coils currently available for the M6, and is one of the main reasons I wanted to try one out. Hopefully, Minelab will introduce a smaller coil for the X-Terra this spring.
Based on the style of hunting I do (old coins at old sites) I would give the "depth of detection" award to the X-Terra, hands down. For VDI, the M6 and X-Terras have a large "easy to read" LCD. The M6 may give more information, but when all the coins are grouped into one tone, I feel it needs to. I like notch discrimination of the X-Terra for many of the sites I hunt, as it allows me to select groups of targets that I may want to accept or reject. The variable discrimination setting of the M6 allows you to set a level which rejects everything below it and accept everything above it. I like that on my analog XLPro as I can learn a lot about a target by watching the subtle movement of the needle. But, I don't like it as well on the digital meters as it is hard to track all those numbers flying by.
So, if I were to rank the Exel series, X-Terra series and Matrix M6 based on depth of detection, sensitivity and visual display, it would go like this:
1. X-Terra 50
2. X-Terra 30
3. Matrix M6
4. ID Edge
5. ID Exel
Just my opinion, so don't jump on me if you disagree. The guy asked for information from someone that has used them all. And I obliged! HH Randy