Others have posted their thoughts recently about the two services and the differences between them, and they did much better than I ever could, so I'll try keep my thoughts short.
Twitter is a great service, when it works. It doesn't take too much effort to find out on Google that Twitter is having all kinds of trouble staying up. And this is a bad thing because people really like the service it offers. You can Direct Message someone, its easy to let people know what you're up to, and you can see what your friends are up too. It's a great way to also play Six Degrees of Kevin Bacon. You can follow your friends post to other people and back others and keep going to the original source. It's been a great way to meet people you would have never had the opportunity too. But it has its limitations, namely that it won't stay working. As well as other little glitches that lets random people start following you.
Friendfeed is a bit of a different service. You can still post updates and follow people, but you can also tie in some of your other Web 2.0, updatable sites. You're followers can know when you've added new bookmarks, new movies to Netflix, updated your blog, and so on, all in one web site. Here's Friendfeed's FAQ that provides a pretty good description of what they do. While Friendfeed has its own faults, such as you can't Direct Message someone and if someone's profile is private its hard to tell who they are, it has some great pluses. It's a great way to roll your multiple services into one location. If you want people to be able to easily follow when you've updated your Flickr account or one of your many blogs, this is the place to see it. Friendfeed let's your followers instantly know when one of these services has been updated.
But what I really like is there's more of a sense of community building. You aren't limited by 140 characters to talk and discuss things. You can have a real conversation and share links and pictures. Conversations are threaded together so you can instantly see when someone else has added a point to a previous post. You don't have to go to thirty different pages and friend multiple people to see where the conversation started or is going, it's all in one neat thread for you. Because I've this I've met and talked to people I wouldn't normally have had the chance too. And they've started following me, which I find really cool as some of them have been big names in library world for a while.
So I'll keep Twitter around, but Friendfeed is really growing on me.
If you have links about the comparison please post them below!