I've used both, coming from a fitbit sense. Hopefully the below is helpful. I started with og galaxy watch, to fitbit versa 2, to fitbit alta, fitbit sense, garmin venu and then back to galaxy watch 4, 44mm. I'm not a gps user, so this did not play any factor on my decision.
Fitbit Sense Pros over Galaxy Watch
Battery life (about 6-7 days for me) vs GW 2 days (fast charging on sense too)
Health Tracking (sleeptracking on fitbit is best in market based on some independent scientific reviews against proper health tracking devices)
small, nice display
GW4 Pros over Fitbit Sense
wireless charging anywhere anytime if you have a reverse charging phone (this makes up for the low battery personally)
fully fledged smarts with LTE which means i can use as standalone for a quick skate around town or run to the shop
addition of body composition and blood pressure measurements
can use an 20mm band
Sense cons = the button is horrendous and the main reason for my sale, so many accidental presses and followed all instructions, would move my wrist the slightest bit and it would trigger the button and whatever was set up with it. Thought it was faulty so returned it and got another, only for the same issue to arise, tried different bands, wearing tighter, looser, up higher on the wrist and made no difference.
GW4 cons = sleep tracking isn't that accurate, misses a lot of the phases. Short battery life and slow charging speeds can be a little annoying, but not that much of a deterrent for me with a charger on hand 24/7.
All in all enjoying the galaxy watch so far and am happy with it over the sense, its better looking in my eyes too with the ability to use any 20mm band.
Happy to answer any questions on both.