Someone recently said: "I would love to see conversion rates to atheism between creationist/conservative Christians (like John Loftus) and more liberal Christians." The implication is that conservatives leave their faith and become atheists more often than liberals do.
By proxy this shows liberalism is a better brand of Christianity. I suspect both implications are correct. Conservatives have already rejected liberalism as outsiders to liberalism, and for good reasons. So once they reject their own conservative Christianity it's probable that a high number of them eventually become atheists.
Unlike former conservatives though, liberals have probably never critically examined their faith from an outsider's perspective, and this makes all the difference. Since liberals believe in less then liberalism is harder to reject, for the less you take on faith the better it is. ;-) Of course, since that's true agnosticism (understood as a position against all metaphysical claims) would be better than liberalism. And atheism (understood as no religious beliefs at all) is the best conclusion of them all.