Essentially, all IM services can be viewed as running off the same idea - instant messaging. Yes, they're all free, yes, they're all received instantly, yes, you can talk to anyone in the world at any time you are both online, yes, you can have a live conversation with a webcam or microphone, yes, you can connect to your inbox. The major decision you have to make is "who do I want to be connected to?" Aside from all the, yes, the cute graphics or the super annoying sounds they make when a message is received or who updates their program most frequently or who has encrypted communication, the most important thing you need to sort out is connectivity.
The concept isn't that hard to get - a service that is "totally free" where you talk to anyone over the internet. The only foundational difference between services is connectivity. All my friends use MSN Messenger; therefore, I use MSN Messenger. Sure, Yahoo! could have x features that MSN Messenger doesn't, but why the heck would I get it if I have no one to talk to? So you want to be connected to everyone, get a convergent IM program. Which one should you get? That's all up to your patience, but the first thing you need to prioritize is your connectivity. Even if the program you choose turns out to be crappy, the only thing that matters is you're talking to the people you wanted/needed to. It's like marriage, it doesn't matter what you're doing, as long as you're with the one you want to be with.