First, the 800/900/1100 seriesis available through 2016 model year to us, which is more than just support.Second the Helix screen is the only benefit as it uses newer technology. If the 859 screen is fine, in our opinion, then go for it is our suggestion. To us the 800/900 model units has been a staple on depth finders. The Helix and core 800-1100 units do give some in DI signal to the Onix and HDS units. Other than this small difference the rest is preference and programming. Our thoughts are no opinion. If is based on your mounting room, placement and preference on what you need.