What, then, is the real difference between A-theory and B-theory? My suggestion is that there is no clear difference, has never been a clear difference, and that the distinction is just not a useful one to make because it can only be made in superficial, purely verbal, ways that have nothing to do with the price of potatoes. That's a highly controversial claim to make; but I've never seen any reason to think otherwise. The original distinction was made in terms of translation, which made sense: it was just a distinction between two views about whether 'past-present-future' (PPF) or 'earlier-later' (EL) were superior ways of talking about time. The A-theorists were ones who claimed that some facts characterizable in PPF format were not characterizable at all in EL format; and the B-theorists held the reverse. These were, it should be noted, not the only possible positions; it is also possible to hold to the position that PPF and EL are, given reasonable suppositions, perfectly intertranslatable, and the only translation difference between them is the commonplace one that for identifying some facts PPF is simpler and for identifying others EL is simpler; likewise, it is possible to hold that each is able to characterize facts that the other is not, and thus that neither was superior. But one can see how the distinction makes a certain amount of sense. If PPF is able to characterize facts EL is not, or vice versa, then those facts in contention could serve to distinguish the two theories. If people privilege PPF over EL in characterization, they are A-theorists; if the reverse, they are B-theorists.
At some point, however, probably with Mellor, it was recognized that, while the translation approach made sense, it couldn't do what it was supposed to do without begging the question. As Mellor noted, suppose we have a statement in PPF format that can't be translated into EL. What difference does it make? A statement that can't be translated into EL could still be made true by a fact that is more adequately characterized in EL format. In the same way, a materialist might hold that while first-person talk can't be adequately fleshed out by any translation into third-person talk of which we are aware, we nevertheless have good reasons to think that all first-person talk is made true by facts most adequately characterizable in a third-person way. In other words, an inability to translate everything sayable in PPF into EL could be due to a defect or limitation in the PPF format (vagueness, or simplification, or whatever). So the translation approach fails to give us an interesting distinction. However, what are we left with? Not much. For we need some non-question-begging way to identify whether a given fact that makes a statement true is more adequately characterized in PPF or EL, regardless of whether the statement (if in PPF format) is translatable into EL format, or vice versa. We have (let us face it) no way of doing this.
I am, I think, much more convinced of this now than I was even then. When you try to look at the details of different theories of time, you find that there are only two things the distinctions among them are made of -- claims about language (which, as noted above, have been central in analytic discussion of time from the beginning but have already been shown to be inadequate) and a sort of picture-thinking, with each major position being constituted by a kind of metaphor or analogy -- the line, the growing block, the moving spotlight, or what have you. And when you try to get more sense of the metaphor, you either get nothing or you just get back to language claims (which, again, have already been shown to be inadequate). So the whole thing seems to be about the best vocabulary to use, combined with arguments that are, frankly, a lot like arguing about whether time is like a river or like a highway or like an arrow, and the more I have read in philosophy of time over the years, the more it seems that way. I'm very much a champion of metaphor in rational discourse, but when you're talking about the real world, you have to have some way of linking your metaphors to the real world, not just to language about it, and disagreement based entirely on metaphors is tricky business.
Since I think presentism, as one form of A-theory, is ill-formed, I disagree with Ed that Aristotle and St. Thomas are presentists. They can be read that way, but that's because in a field of ill-formed positions they can be read any which way you please. But I also think the arguments against it all fail, for exactly the same reason. After all this time, the only position on time that still stands is perhaps that of St. Augustine (which I discussed even earlier here at Siris).