Context:
I've used my Forerunner 645 (no music) for over two years now, when I bought it I lived in a flat city.
Now I live in a hilly countryside.
I run about 3000k per year, so use the watch a lot, and love accurate metrics.
I've been lucky with the 645, in that I've had very very few issues, esp since firmware 6.70.
Last week I finally bit the bullet and bought a Fenix 6S Pro, and thought I'd share my notes here.
- The black metal exterior is ok, but the 645 looks better. It doesn't scream "rugged sports watch" at the world, and still it has survived many adventures in the Alps.
- Watch strap - the Fenix strap doesn't quite fit me right, it's either too tight or too loose. I'll look at maybe swapping it for my old worn out strap from the 645, or another custom strap.
- Wearing comfort: the Fenix is heavier. It is less comfortable to wear, but it's not the end of the world.
- Topo Maps - absolute game changer for trail running (2x/week), that was my main reason for upgrading.
- Music - a nice addition for speedwork or track runs (2-3x per week). A nicer metronome, and I can now do this without a phone.
- PulseOx: I live below 850m, so I'm still not sure what it's good for since "this is not a medical device". Enabled during sleep for now, I might disable it at some point.
- Optical HR: having run with both watches, the accuracy is similar, which is to say, it's very good unless I run intervals.
- GPS accuracy: the Fenix has Glonass enabled by default I think because the Sony GNSS chip is tuned for it. The 645 was noticeably better at this with GPS only or with Galileo added, the track matches the map more closely and there's fewer random readings when I run along a wall. Overal distance is short in both cases, by 1-2%. I suspect Garmin's post-processing cuts out some distance to resemble race distances more closely, rather than give you the exact distance travelled by your feet.
- 3D speed and distance: this is new. I've enabled it, it's nice plus to have in hilly country.
- Barometer: both work well for me
- Sleep: the 645 was good at this. The Fenix thinks I wake up 4-6 times per night and cuts my total sleep time by 30-120 mins. It's a joke.
- VO2max: the Fenix now accounts for elevation and temperature, and enables the estimate for trail running. Total winner.
- Body battery is nice, but easy to live without.
- PacePro: I get the appeal, but it's only really good for races as you cannot navgiate at the same time.
So for me, it really comes down to topo maps + updated VO2max formula vs accurate sleep measurements + better comfort.
I'll end up selling my 645 and missing the sleep metrics and comfort. The 6S Pro is not a perfect replacement but it's better for what I do with it.
Hope this helps some of you considering the upgrade!
