They both have their pluses and minuses - twitterfeed only grabs up to 5 posts per hour, so if you're pushing more content than that, it won't go out. It also only grabs your feeds on the hour, and depending on your publishing schedule, you might not have something go out for an hour or more. It just works out that way.
Twitter tools sends EVERYTHING out. It also has several other features that are pretty nice. But yeah, it all depends what you want to do. I've found TwitterTools to be pretty reliable.
Another good one is Tweetmeme, which is kind of like a retweet button you can put on your site