On the whole, I think it's easier to persuade a conservative theist to become an atheist, than it is to persuade a liberal atheist to become a conservative.
The reason is pretty straightforward: while it's a somewhat involved process to clearly demonstrate the flaws in liberal thinking, it's quite easy to demonstrate the blatant contradictions in religion. Essentially, religion is the more vulnerable position of the two.
Of course, this assumes that you are talking to someone whose attitude is sufficiently reasonable to be amenable to persuasion. The nut cases on both sides don't qualify. Fortunately, the majority of people don't seem to be quite as hard-line about things.
I find them equally difficult to persuade but in my experience conservative Christians are more amicable debaters.