Fenix 5 vs Vivoactive 3

Battery life - I used mine for a 4 hour obstacle course run on Saturday just gone (more than 48 hours ago now). The battery was at 75% before the event and checking it now it's sitting at 39%. You'll be fine for a 10 hour event. The 20 hour stated time is somewhat ambitious but anything around 12 hours is going to be fine. It also doesn't take very long to charge.

Bluetooth - there's been lots of connectivity issues with the Fenix line, although mine isn't as bad as it once was - it does now stay connected when within reach of my arms, so in your pocket should be fine. Also if you have an iPhone iOS 11 introduced a number of issues with the Garmin app not being able to stay connected. on iOS 11.1 and the latest update of the app I'm seeing fewer and fewer problems, though I do still have to occasionally force quit the app to make the watch reconnect (even though the watch will say it's connected) to the app and my iPhone 7. I don't have to do a full reset of the phone network like it would appear some people do.

Sensors - are really hit and miss. Any sensor that is more than arms length away from the watch is an issue for me, even Garmin ones. I have one of the very first silver non-sapphire models though so it's entirely possible I've ended up with a early hardware dud. There does seem to be less people complaining now, so it's entirely possible Garmin have quietly made some hardware revisions.

I went for a 200km ride over the weekend (about 9.5 hrs total, incl stops), and went from about 95% to 53%. Some backlight usage in that.

I also find the BT connection is more reliable than what it was. Of course, BT connection comes down to the antenna design and BT chip in the phone, as well as the watch.

Sensors - this is a definite issue with the F5. While the F5 supports a wide range of sensors from a software standpoint, there is some interaction between the hardware design and the ANT+ chipset that causes some issues with some sensors. Garmin's unofficial comment to a well-known tech blogger on the situation:
“We are aware that there are reports of ANT+ reception issues, particularly with some specific devices. Garmin is looking at ways to improve the performance, but there does not appear at this time to be a “quick fix” software update. We do anticipate future designs to improve support for identified third-party devices that are not consistently meeting expectations today.”

I'll be honest - on the sensor connectivity standpoint, you are probably better off with the Forerunner 935, which has the same internal hardware and software as the F5, but a different external case that seems to cause fewer connectivity issues

I had a Vivoactive HR and was considering a VivoActive 3, however bought a a Fenix 5.
Pro:
1 - Battery duration. By specs in the fenix it is the double. In fact I can get easily 1 week, I don't do 10 hour ride but 6 hour;
2 - Less bugs. During the VivoActive HR had a lot of bug problems, since the Fenix is a more mature product the most severe bugs are ironed out.
3 - Construction quality and looks, maybe an unreasonable pro .
4 - A simple map that allows me to not use GPS in a lot of situations;
Cons:
1 - Since my main focus is MTB and don't have a power sensor that means that Fenix Vo2Max doesn't mean a thing (since then started to run :) )
2 - An expensive piece of hardware in the trails;
3 - A nidicolous BT and ANT+ range maybe half of the VivoActive. However they are, in my case 5 meters and that is enough.

Things that are the same in both:
1 - Sensor pairing: TEMP, speed sensor are a pain to connect. HR and cadence are perfect (don't know why).