Why would anyone buy an etrac over the equinox
Because we prefer digging silver, not iron. Just saying.
Because we prefer our choice of after market coils. Just saying.
Because we would rather not need to buy an after market shaft and worry about the coil ears, because they are flimsy on the Nox. Just saying.
As others have said, it depends what your are looking for and where you are hunting. I hear the Nox is really good at finding nickels. I suppose if you want to find nickels, the Nox may be your machine.
For me, I like hunting huge fields and hunted out parks. This indicates a huge coil and tuning the machine to give you a clue if a deep target is iron. The E-Trac allows me to choose any coil I want, and not worry about the coil ears breaking, as mountains of Nox users have reported when using the only large coil they are permitted by Minelab. Moreover, the E-Trac has settings that allow you a pretty good chance at figuring out if a deep target is likely to be iron very quickly. As far as I can tell, my Nox does not have these settings, and deep iron sounds like deep silver, at least to me. Online videos show all the hoops Nox users use to determine if a target is iron; if you time these swing this way and that way and change all the settings, it is just faster to dig. Problem is I'm old and can't dig every target.
I don't know your situation. Best advice, if you have the budget, is use both machines in your situation, and make the determination yourself. That is what I have done, and I am sticking with the E-Trac, using the Nox in only very specialized situations, like very trashy ground around old farmhouses, where the good targets will be shallow, and the Nox's improved separation will give a slight advantage over the competition.
YMMV of course.