For me, the hot money right now is on the older edge 800s. it's essentially the same as the 810 ( 810 has a slightly better GPS chip, and connectivity to your phone to upload rides and what not.), but unless you want to be constatnly draining your battery on your smartphone and have people track you and have all your rdies uploaded where the world can pick it apart, there no real point aboout the newer 810 versus the 800. you can piuck up older 800s NOS for a good price now.
That is the feature I enjoy the most out of my 510. My wife can track me and knows when I am heading home so she can gauge how far away I am from arriving. Very useful if we have plans later that day. Also for her own sense of concern as she knows where I am incase something ever happened to me.
I also really like that my ride is uploaded before I am even in the house. No need to turn any computer on. No need to find a cable. No need to open a program. I am done. Time to move on to the next thing.
Also the battery draining isn't an issue. With data streaming on phone, blue tooth and gps on my phone probably averages 6-9% battery use per hour while connected to my Garmin. Not bad at all as I never leave house without a fully charged phone.
If you are concerned about phone battery draining, why would you want to use your phone as a bike computer with ANT+/BT 4 adapter connected to it? I would want those two things seperate.
in the coming years I won't be surrised if smartphones supplant cycling computers. you could easily put in a GPS, ANT+, bluetooth lite into your smartphone and build a bikemount for it, then write a training app. in fact now that we have things like strava, I don't think it's very far away. someone just needs to build a killer training package into a phone for biking into a cleverly sized smartphone and I think cyclists would be all over it.
Smartphones already have GPS and many of them already are ANT+ capable. Just buy the Garmin ANT+ adapter for your phone. Plenty of cycling apps for the phone you can use at this point if that is your thing.