I would suggest you definetely not to go for Line 6. It makes sense only if you want to use teir guitar amp sims. But if you want to use it for something else it sucks. I could neber get reasonable latency out of it, also the sound recorded is really "plastic" I now look myself on M-Audio C400 or C600.
I also used to have mbox and also did not like the sound when recording with mics.
I had reasonable mic sound when I was recording with EMU tracker pre. Actually I think this was one of the best affordable interfaces. Good quality mic preamps and excellent converters. The problem with that one was, that when I changed my laptop to a new one I started to experience sound dropouts when recording, and a lot of distortions during playback. Chipset compatibility issue. I found on forums that many people face the same problem with EMU interfaces. They are excellent when they work, but you may have troubles.
Buy the way there are good deals now on ZZounds for M-Audio interfaces M-Audio Fast Track C400 USB Audio Interface M-Audio Fast Track C600 USB Audio Interface