Grant Petersen at Rivendell has written a lot about this (he sells only steel, so he's got a point of view, but he makes sense). I have two of his bikes, an Atlantis and Rambouillet, and I can't think of anything I'd change.
I've only ridden a couple of Ti bikes, and both were too small for me (I ride a 64-65cm), so I can't really judge. FWIW, though, I saw nothing to justify the extra money..
Others will disagree, but I wouldn't buy a carbon fiber bike. The relatively small weight savings (small relative to the total bike-rider weight) isn't important to me, and I still have reliability and durability concerns. A neighbor of mine is an engineer who's worked a lot on CF parts for cars and aircraft. One of the things he found was that while they're enormously strong and light, when they finally fail, they tend to go without warning. Steel will show cracks and wrinkles and hang together for awhile. May not be a factor in practice, but every time I watch a fork flex, I think of it.