As @Totem Polar noted, your U1 has an ETA movement (the 2824-2, IIRC; the new ones have a Sellita SW 200-1, which is more or less a clone of the ETA). This is an extremely common movement, and there is no real reason that you would need to use an authorized service shop or send it all the way back to Sinn itself if having the movement serviced is all you need to do. This is partially why I somewhat regret getting the U2 S, as the additional features are cool (such as the argon purge and the desiccant capsule), but require the specialized shops to do it. For a more simple watch like the U1, I would not let the difficulty of getting it serviced by authorized shops or the manufacturer to be a deterrence, it would be akin to not wanting to buy a Toyota Camry because your local Toyota dealership's service department is terrible, ignoring the fact that the Camry is widely used and any decent auto shop can maintain/repair it.
I'm basically at just three watches these days, and can easily par it down to two:
- Garmin tactix Delta
- Sinn U2S
- Tag Heuer Carrera Date
The U2S only sees use in a closed area, where no personal electronics are allowed, and I could ditch that pretty easily.
For me, the Garmin provides a huge amount of utility. It provides phone notifications (damn nice little feature once you get used to it), along with navigational abilities, some basic health monitoring, exercise tracking, along with all the standard watch stuff (timekeeping, time elapsed, date, etc.). It's definitely not something that will last forever, and is semi-disposable, but is useful enough that I am happy to buy new Garmins as they wear out, even at the price they ask.
The Garmin is definitely a tool, though, and for when I need something much more jewelry like, the Tag is what I wear, as it is very much a dress watch. The Garmin in informal wear simply seems ridiculous, and is probably stuck at smart casual at best; it still looks a bit out of place in business casual, especially as beat up as mine is.
If I had to choose just one, I would pick the Garmin in a heartbeat, and just go watch-less when a dressy appearance is the primary concern.