I own a Nox 800, Nox 900 and the Legend. I am one of those rabid followers that markinmichigan mentioned but I am rabid about all three.
For me the Equinox 700/900 are setup from the factory with more aggressive factory default programs than the Legend. If you are new to metal detecting that might be a bit overwhelming. If you have detected before it shouldn't be a big deal.
The Legend has proven to truly be waterproof. The 700/900 hopefully are Minelab's redesign to fix some of the leaks encountered by many Equinox 600/800 owners. They have had three years to fix that issue so, we will see.
I like the Equinox user interface better than the Legend user interface.
The Legend has a full feature set that is comparable to the Equinox 800/900 so it has several features that the Equinox 600/700 lack for a lower price than the 700.
Detecting with them side by side in the high iron mineralized ground where I usually detect, depth is virtually identical using the same sized coil and with similar settings.
You can't go wrong with either the Legend or the 700.