A pick will give you a more mid-range rhythmic sound. If you use your fingers, you'll get a fatter and boomer sound. I think the pick sounds better in rock situations, and the fingers in more of a jazz or jam situation. So when I play, if the guitar is distorted I'll use a pick, if the guitar is clean I'll use my fingers. To switch all you do is tuck the pick into the palm of your hand with your thumb. Not hard to do. In the end it just goes by the sound you want.

The best thing that will come from playing bass is you'll get a chance to look at music in a different perspective. By playing many different instruments, you get to see how they work and how to make them work with other instruments. See you can't really write a piano piece if you don't play piano. Yeah you may be able to throw something together but it's not until you sit down and play that you actually see how the instrument works.

I don't think your guitar playing will get much better from playing bass. You'll probably learn to pay more attention to a groove rather than strumming and palm muting chords, but it won't better your technique on the guitar. It will enlighten you, and maybe when you write your own songs you'll better know how to write bass lines with guitar parts. They are two different instruments which focus in different areas of music. That's where you'll benefit.

I would listen to some good bassists from the style of music your going to be playing before you try out. Maybe even learn some of the tunes (better yet the ones you know on the guitar already). That way you see how to put things together and you don't go in playing bass as a guitarist. Instead with some knowledge of bass playing.

One more thing, listen to the drummer when you play. If you sync in with him, your band will sound alot better.