UK Brian has this right. Make your own test bed and keep notes on different machines.
I've done this for years and it really does separate myth from reality. I never take a poster's word for it on depth. I like to read their thoughts, but only my own tests do I believe. Following are some common machines I've ground tested on the same 12" deep quarter in my Florida soil which is fluffy and without mineral problems. Denser soil does kill depth. Minerals are not anything I've ever had to deal with because I've not used a detector above Carol City near Miami.
All on a 12" clad quarter:
Fisher 1212X - no hit
Fisher F-75 - no hit
Whites MXT - chipped signal, will hit at 11"
Whites Classic III - no hit
Minelab Soverign - no hit
Minelab E-Trac - chipped signal I would not dig. (tuned this thing to death)
Tesoro Vaquero - not hit, will hit at 11"
Tesoro Deleon - no hit
Tesoro Cibola - strong hit. will detect a quarter to 13 inches and continue to chip at 13.5"
Ace 250 - please! this thing wouldn't detect an anvil if it fell on it.
Whites new Coinmaster - would require the same anvil to strike dead-center. pure junk
It is my finding that high cost is not an indication of depth and the Tesoro line is very, very underrated if depth is what you want for little money.
OT