That's a tough question. I would have liked to have been able to pick "they both have their place". I prefer e-mail, but send many instant messages too. I send instant messages when I want a quick reply to want to leave a "message in a bottle" for someone and don't care if I get a reply back. I also don't check my typing in instant messages. I'm much better about the effort I put into composing e-mails, and usually expect a reply although I understand it may not always be an instant reply.
I almost always prefer email. As Ray says they both have their place but I'm pretty much a 'solo tasker'. I do one thing well at a time sure I may have 6 different apps open at once, writing a blog post in one window, a forum post in another, and monitoring two servers memory usage in yet another window but for the most part I'm only concentrating on one actual task at any given time.
I often view instant messages as an interruption. It's sort of like the phone ringing although you don't have the option to let the machine pickup because the guy on the other end already knows that 'your home' because you're marked as available. If you're busy and don't answer the person you have a good chance of pissing them off. If I set my status to busy and people still insist on IMing me for no good reason, chances are I'm gonna get pissed off it's a good way to start an argument with someone over something stupid. For the most part I don't sign into instant messengers unless I've already set up a time to meet them via email.
Now Twitter is interesting, I at first discounted it as a fad but it's like a hybird mass email/instant message. I've been playing with it the past couple of days it's pretty cool.
That's a tough question. I would have liked to have been able to pick "they both have their place". I send instant messages when I want a quick reply to want to leave a "message in a bottle" for someone and don't care if I get a reply back. I also don't check my typing in instant messages.