I'm thinking of getting a Gear S3 Frontier and use the LTE.
I've got the Frontier, non-LTE version.
Quick review:
GPS works nicely within the city, but if I go out into woody and hilly parts for runs, it seems to be hit or miss at times. Works great with Bluetooth headsets, if you want to leave your phone at home for exercises. Wifi works great as well, I connect to my gym's wifi, and stream Spotify instead of having MP3's on the watch when I'm at the gym (leave the phone at home then as well). I also noticed if I'm at the gym with the Gear S3, phone's at home, if I get a notification on the phone, it's still sent to the Gear S3.
Spotify app can act as "remote" to your phone or computer, or stream directly, but as of yet does not have offline playlists, so if you want to use Spotify, Wifi is a must (or Bluetooth if you just want to remote your phone I think).
Does not have continuous HR monitor, instead checks your HR every now and then during the day. Checks much more often if in exercise mode (if you start any exercise with the s-health app on the watch, or if it detects an exercise).
Has a good range of exercises in the s-health app, but does lack some (no weight training, and selecting "other exercise" disables the HR monitor, so "other exercise" is sort of useless, I usually select Yoga, just to get the HR stats).
I like the fact that I can dismiss any notification on the watch and it's also dismissed on the phone. Also, I can select, "show on phone" on notifications (emails, sms, etc), and it auto-unlocks the phone and shows the notification.
The floor counter is a bit more iffy than on the Fitbit, seems to miss floors every now and then, seems to be extra sensitive if you don't move your arm as much when walking up stairs, compared to the Fitbit.
The S-health app all in all seems more feature filled compared to Fitbits app, it's also got better food database as far as I can tell. Though it cannot sync with Withings scales.
There was another downside to it I was going to write, but I lost track. I'll add it later if I remember.