Coming from a fenix 5 I have to say that this is an outstanding device. Sure it has a few bugs, but so does everything else out there. I used to hate Garmin. I wanted to smash my fenix 3 and fenix 3 HR with a friggin hammer many time because of all the bugs, but Garmin seems to be stepping up their game and fixing many issues. The fenix 5 is a great device, but I think it's too advanced for me. I know how to use all the features, but I find myself obsessing over features that I don't really need anyway. I am not a pro athlete, so I don't need advanced running dynamics, or lactate threshold, or two levels of training effect. VO2 max is ok to know so I can gauge my fitness level and improvement over time, but all I really need is my heart rate, distance, pace, time, and calories burned.
I tested my vivoactive 3 today comparing it with my trusty Suunto Ambit 3 Peak (extremely accurate GPS & heart rate strap). I used the heart rate strap on the A3P and the optical heart rate on the vivoactive 3. I did about a 7 and a quarter mile power walk/run interval on very hilly trails. This is a good test for heart rate monitors because the heart rate is constantly changing.
The vivoactive 3's optical heart sensor is probably the most accurate I have seen thus far. I kept the screens on both watches on the heart rate screens and checked them very often, and they both matched exactly most of the time, and the few times they did not match they where still within 1 to 2 BPM of each other.
The GPS track was only a tenth of a mile different between the 2 devices, and that was across over a 7 mile distance.
here is a link to the workout...
https://connect.garmin.com/modern/activity/2220516721
I don't think you're going to find a device for 300 bucks that has more to offer than the vivoactive 3, and yes, I did five the fitbit Ionic a try and it's basically a water resistant Blaze with GPS and some smart features (some of which don't work yet). It did have a very nice screen and was pretty accurate though.
This is only my opinion, and yours may vary.