I havent owned the Um but I can say a few reasons why I dont:
1) I don't like the rubber on the studs--yes it can be taken off but its not something i like to do to a 400 dollar knife
2) lack of pivot bushing. This was the big deal breaker to me. I hate blade play and I hate to have to play with pivot tightness--things I don't have to do with my seby. Im sure it is not bad on the Um, but it is something that gets worse over time and I find it a huge downgrade from the sebenza
3) The grooves. Sure they might add grip but I find they look kind of silly or childish. Remind me of the grooves on the bm mini skirmish. I just don't find them very classy looking.
Things I like about the Um
1) blade shape and handle shape. Looks like it would be comfortable and I like the blade.
2) looks pretty cool
Overall, you can see I am not a huge fan of the Um. I was so excited when CR released a new knife and it was a big dissapointment to me. Im glad lots of people like it, but it isnt for me.
Ill suggest going with the Seb. Its a classic and stood the test of time. IMO you can't go wrong with it.
Oh and to answer your other question about scratching. The regular seb does scratch pretty easily but it looks cool. The more expensive ones will scratch as well. They all have titanium slabs as their base, the difference is in how they are finished. The standard sebs are bead blasted while the more expensive ones with inlays (except the micata which is the same as the standard) or graphics are polished--they are still titanium. You can always get a spa treatment but I think that destroys the story of your knife which I think is a total waste. Love your knife, use your knife and respect your knife. Don't worry about little scratches, they add character. Let your knife age with you
Good luck!