I honestly haven't tried the V800 but I could not be happier with my 920XT. Love it, love it, love it. The one sort-of negative is no navigation, but seeing you already have a bike computer, you already have that covered. The 920XT finds satellites and connected devices very quickly, is spot-on accurate (I think) with distance, is correct with how many strokes it takes me to do a swim lap, etc. I find it easy to navigate through the options/settings. One thing that I didn't expect to enjoy so much is the optional metronome to help me time my strides. Also, if you get the HR monitor bundle, you can see your cadence, vertical oscillation, and ground contact time. That might be more bells and whistles than is needed, but it really helps me analyze my form quickly during runs.
But I agree with the earlier poster, check out DC Rainmaker's blog and you'll get more than enough info on both watches.
https://www.strava.com/athletes/10327392