Have to agree that when it comes to guitars a mic'd amp is going to sound 'better' than going direct in to your machine.
The reason for this is, apparently, a guitar amp is designed to do a certain amount of processing to the signal (something with the higher frequencies, i think?) which is lost when going direct in to a mixer/interface. So going direct in never sounds like a an electric guitar should because you missed the all important amp processing.
The bass is a different story though. You can go direct in and get good results. I think you'd need a DI box between your bass and the mixer/interface though unless you have a hi-z input on it. Some people, myself included, like to go DI and mic their bass amp when recording so you end up with 2 tracks which you can blend - a good clean signal and a mic'd track that you can use to get some of the amp/room sound.
Hope that helps?!