Fenix 5 owner here who recently upgraded to the Ultra. For daily stuff and "normal" workouts, it's way more useful. I get alerts from my cameras with useable images when they detect faces around our house, for example. I can do useful things like watch my grill temp + probe temp from the watch. The face customization experience is a LOT better than the Fenix. It feels like an entirely different class of device with the responsive touchscreen and bright screen that refreshes quickly. The Fenix feels like a souped-up version of the Timex Ironman Triathlon watch I had in the 90s (with Timex Datalink to sync data with a PC via CRT flashes!
https://en.wikipedia.org/wiki/Timex_Datalink ).
As far as which I'd take backpacking: neither. The Fenix isn't a better UX than our iPhones, and we already take battery backups for the phones + put them on airplane mode with low brightness and get ~2-4 days of battery life out of them that way (usually shutting them down completely at night). The watches aren't worth the grams yet -- it's weight I'd rather spend on a nice phone pocket for my backpack (I like the Prometheus Design Werx SPX Pouch). I'm hopeful that another year of software updates for the Ultra might fix that -- if it did have useable topo maps + dynamic offline route planning, it might be worth it.
The other big benefit I see is cellular. Now, when I go for a run / bike ride / etc I don't actually need my phone, and if I'm confident that I might stop somewhere with Apple Pay, I might not need my wallet either. Back when I was riding pretty seriously, it was common for folks to just bring their ID and some cash. I'm also excited about the prospect of using the watch as a fully-featured bike computer, given it's about the exact same size as the old Polar bike computers I used to love.