I wouldn't stay at a hotel within walking distance of the United Center. It's really not a good area. However, you can stay at a downtown hotel, which results in about a 5 minute drive. Plus, there are a ton of great places to eat at within walking distance of the downtown hotels, which isn't the case around the United Center.
I stayed at the Cass Hotel, and I know a lot of other people from here did. Literally everyone I saw at the hotel was wearing a U2 shirt
It's not the nicest hotel in the world, but it's clean, relatively inexpensive ($80 to $90 a night), has an on-site garage (for a fee), and is ideally located. Here's their website:
http://www.casshotel.com/
Staying at that hotel puts you at about a 5 to 10 minute drive (depending on traffic) to the United Center, 2 blocks from Michigan avenue, half a block from the best pizza in Chicago, 3 blocks from the best breakfast I've ever had, and 2 blocks from one of Chicago's train stops that will take you pretty much anywhere you want to go.
If you're looking for somewhere nicer, there are a ton of other hotels right around the same area too.