This is going to be the most absurdly oversimplified statement I can make, but it is pretty close to the truth.
In VLF you can buy four basic detectors:
1. Low frequency detectors that cluster around 6 - 9 kHz. These detectors tend to be forgiving of ground responses, do very well on coin type targets, and are not as "sparky" or "noisy" in general operation. Good coin machines.
2. Mid frequency detectors that tend to cluster around 13 - 19 kHz. These detectors are hotter in general, more responsive to ground mineral variations but also better on small targets. Good general purpose detectors.
3. Multi frequency detectors. Best feature is ability to handle salt water and ground mineralization at the same time, superior beach units. Tend to have good target ID accuracy. Generally designed for coin detecting and so act more like the low frequency machines than the mid frequency machines. Quiet on the ground, not too hot on tiny stuff.
4. Very high frequency detectors 30 kHz and higher. Niche gold prospecting machines.
In each category performance is very competitive. The least talked about and most important difference regarding depth between models are the search coils employed. A detector that comes with a 10" concentric coil will have markedly different operating characteristics than a 10" DD coil. DD has become a bit of a fad these days. People talk about knife like shooting blade detection patterns, all I can say is they have never put a coin under that coil and observed reality. Much that you read is marketing hype when it comes to multi frequency detectors and DD coils.
So my advice is first look at the category. You can usually ignore the high frequency units. If you decide on a particular category, most machines in it will be comparable in performance with the biggest thing being the coil differences.
My point being that yes, to a large degree, most mid frequency detectors, for instance, have pretty comparable performance. I can grab just about any of them, put the proper coil on for the situation, and do well. Like many people I tend to obsess endlessly but to a large degree uselessly over performance differences. In VLF detectors these days I am just trying to decide if it is the Chevy or GM model I like the most.
Hope this helps. It is as I say a simplification but sometimes simple is what we need to make sense of the complicated mess the metal detector world is becoming.