All in the coil IMO. Etrac or SE Pro has a 11" coil on it. I can say my GT with the stock 10" coil is deeper in my ground than my prior 3 Explorers using 10" coils were, but those coils I hated on my FBS units. The 10" Tornado IMO is a much better coil with a more "tight" field. The 10" FBS coil's field seemed rather "fuzzy" and I think just sucked up too much ground stew in *my soils/sands* minerals I suspect, costing me depth or stability issues. When I pick up a SE Pro here down the road to sit along side my GT I'm throwing a 12x10 on it and getting the 6" Excelerator for my super heavy trash coil.
In head to heads for the past two years before digging targets, a few friend's Etracs have yet to see something any deeper than my GT with the 12x10 on it. Both seem equal in depth thus far in our soil, and besides the casual comparisons before digging, the other reason why that's the way it looks in our soil is a few times I even could hear a super deep whisper and when he came over and checked it he got no response. This is using minimal iron rejection much lower than what the GT has built in and an overall open screen otherwise, and best overall settings most tend to agree on to max out their performance.
The difference? I was running full blast manual sensitivity since the 12x10 often allows that even in my soil. He was running Auto+3 and that setting was very near full sensitivity, so a few times when the above happened I'd say "take it out of Auto +3 and raise it the last few clicks in manual to full blast sensitivity." And then he could hear them too those few times when he couldn't at first, so that tells me both machines are about evenly matched in depth in my soil, and largely depends on coil choice as to depth and such.
Also, all on edge or very badly masked (or combos of both) coins that would null for each of us but only at one very tight angle....Both machines have seen just as well every time we've compared targets one of us or the other found first and then would call the other guy over. Now I'm taking to pulling my headphone plug off just so we can get an even better idea of contrast between them. So far no clear advantage to either, and is one of the reasons why I feel the 12x10 has done wonders to make the GT hold it's own against other flagship machines or that fantastic 11" Pro Coil the Etrac and SE Pro come with. Both that coil and the 10" Tornado are the two best *stock* coils I ever used on a machine over the years.
Tip On Depth Meters- I've owned plenty of machines that had a depth gauge on them, and still found I'd just go by the sound of how deep it was to judge it's depth. More reliable to me that way. For example, I've had tiny little targets on a machine read 7" deep or such, but yet I knew by the audio traits as I swept over it it was something small and shallow.
One perk I did find use for a depth meter for though was on my QXT Pro to fish out coins next to a large piece of junk. If I got a coin hit at the edge of it and wasn't sure by other ways if it was real or not, I'd pin point (get a depth reading) of the large piece of junk, and then I'd pull the coil away from it and over the coin hit. If I could see the depth get deeper as the coil left the junk and then get shallower again as it went over the coin hit, then I was pretty sure it was a second target (coin) next to the junk and not a phantom coin hit. Worked great for checking false iron coin spikes as well where I wasn't sure if it was just a nail giving me the coin hit or if it was a real coin next to it. Mostly though I would use other ways to judge the coin hit, by like sweeping at various angles to see if it was still solid or got iron ghostly on me.