I was wearing Instinct Solar watch for about a year now, and while I planned to wear it "forever", some of its issues and unkept promises started to bother me. Also I wanted to return to wearing my collection of mechanical watches, so needed something "low profile" to wear on the other hand. This is where vivosmart 5 came to the play. :)
I don't have long term experience yet, but I can probably quickly asses some points and compare both "watches" for anyone who's interested.
Vivosmart:
- vivosmart is quite small and as long as I disable display gesture, it looks just as a generic black "fashion" band, so IMHO it's fine combination with a mechanical watch
- I bought S/M version (male with about 18cm wrist) and I have only about 5 holes left on the strap. I like that it doesn't overhang as most of the straps, but would probably prefer the buckle to be more in the middle of my wrist. L version would probably suit me more with the position of the buckle, but the extra strap length would come in the way in my daily activities. Garmin should provide both straps in the box so users can choose whatever suits them more (and even better, S, M and L variants).. 
- I don't really care about the size of the display, I use it to quickly glance at the heart rate and start/stop the activity, which I can do quite comfortably
- OLED is easily readable, but PWM used for screen dimming is annoying due to its very low frequency. Fortunately I don't need to use it often.
Advantages over Instinct watch:
- much smaller footprint
- reacts faster to HR changes (when I start/stop moving etc.)
- extra readings (breathing rate, VO2 max)
Disadvantages:
- no always on display + OLED technology (I like LCD more)
- touch screen (buttons are easier to use, but it would be next to impossible to put all the buttons to vivosmart )
- battery life (I get about 2 weeks between charges on instinct with occasional GPS activity)
- no beta firmwares, not possible to upgrade/downgrade manually
What I don't like on Instinct:
- solar feature is basically just a gimmick, it's completely useless indoors and has minimum impact outdoors (unless someone really spends almost whole day outside). I was expecting better efficiency, at least in "basic" watch mode (HR + sleep monitoring).
- while it's possible to customize the watch face, options are quite limited and at the end not what I expected - Garmin refused to provide even basic date formats like dddd.dd.mm.yy(Instinct 2 should be much better in this regard)
- GPS accuracy is comparable to my phone (which I have in my pocket/backpack anyway) and consumes battery quite fast. It's nice to have this possibility, but not a huge advantage over "connected GPS" of vivosmart. I need to charge my watch or phone, so it comes basically to "pick your poison".
- I hoped to use barometer for weather predictions, but it's not really possible. Altimeter during activities differs too much from the real altitude (even when calibrated on every stop with known altitude), so I question its usefulness. Also it resets to wrong value every night and Garmin refused to fix this behavior (or even admit that it's completely stupid idea).
At the moment I am debating myself if I should keep instinct watch for outdoor activities, or just get rid of it completely.. Anyway, hope that someone finds this post useful.
