Most of the time I would say 1. location, 2. operator, 3. detector.
So, taking Maxxkatt's example, an Ace 250 run by an experienced user and an Equinox 800 run by a novice to intermediate user with similar sized coils, in good dirt on well spaced surface to 6" coin and jewelry or relic targets, I would call that a toss-up or slight advantage to the Ace 250.
On deeper targets, if the Nox is setup correctly, the Nox will win that if the less experienced user has a clue about what deeper targets sound like. A more experienced user of the Nox would easily find 8"+ targets if there are any and depending on if they are non-ferrous without too much iron contamination the Equinox will correctly identify those 8" to 11" coin sized or bigger targets.
Add in lots of aluminum trash with multiple surface to 6" non-ferrous targets always under the coil, the Nox with its much better target separation and recovery speed will win that contest if the users have fairly equal skill levels.
Change the scenario to moderate to really high mineralization or iron infestation on ground with good targets down to 8"+ and the Ace 250 will lose big time against the Nox 800 along with most other single frequency detectors with users of the same skill level.
At a saltwater beach and surf..........the multi freq. Nox is hard to beat without going to a CTX, Excal, Sovereign or PI detector.
Simultaneous Multi Frequency detectors like the Equinox, Vanquish, Etrac, CTX 3030, and to a lesser degree the Whites V3i and the DFX really do offer an advantage over single frequency VLF detectors for target ID accuracy on deeper targets and for detecting targets accurately in more difficult soil/salt conditions.
So, in moderate to extreme soil, salt conditions or extreme aluminum or iron trash: 1. detector, 2. location/operator
Jeff