The great thing about Portland is that it is really compact and easy to get around. Most of the hotels are bunched in the core downtown area, which is just across the river from the Rose Garden Arena. About a 5 minute drive. There are a few hotels right near the Rose Garden, but I have not stayed in them so I would not want to comment. If you do not mind being in that core area that I mentioned, then try the 5th Avenue Suites. Even though we live here, I have stayed there twice and it is really a nice hotel with a great staff. It is a Kimpton property, like the Alexis in Seattle that you probably know.
I was just at the Rose Garden for a concert this week. You did not ask for restaurant recommendations, but if you are interested, we had a great casual dinner at the Widmer Gasthaus and it is only 1 mile from the arena.
Have fun at the concert!