I like the Deus in thick iron. Equinox in wide open sites, beach, and in inclement weather. The Equinox may ID targets deeper in high mineralization than Deus and may be deeper overall in mild ground. But nothing separates from iron better than Deus and it may sound off on targets deeper in mineralized ground than Equinox, but it will not provide an ID or may sound off as iron. PI is usually the best way to go in super hot ground. If you hit a site repeatedly, hitting it with different detectors on different visits can be productive, so having a backup detector to provide that capability is usually worth the investment. The Equinox is a great compliment to the Deus, but not necessarily the only option (see Smokey's Tesoro suggestion if you want to keep it light and simple).
I think the Equinox strength lies in it's versatility. Deus bests it under certain conditions, but it really does a lot of things well and some better than Deus (e,g., saltwater and wet salt sand performance and freshwater performance without Deus antenna claptrap are Equinox strengths over Deus), gold prospecting is probably a wash, relic hunting depends on conditions, more reliable target ID on Equinox, audio is good on both and wireless, and Deus wins handily in the weight and portabilty department. Accessory coils are naturally cheaper on the Equinox as Deus crams the brains in the coil which is what drives coil cost (not really driven by the wireless coil aspect).
I like it as my compliment to Deus snd have had great success with both.
HTH