A colleague asked me: Which is correct, "premier lawyers in the country" or "premiere lawyers in the country"? I was inclined to say "premier" for "foremost," and save "premiere" for "opening night." But a quick dictionary check suggested that "premiere" could mean "foremost," too"; I saw this both in the American Heritage and in the Oxford English Dictionary. So I don't see how one can say that either is "incorrect."
But my colleague, I thought, didn't really want to know which was "correct"; he wanted to know which was better. And for that, it seems to me the answer is the more common term, which is less likely to be jarring, confusing (even briefly), or perceived (even wrongly) to be incorrect. So I Googled, and it turned out that my initial inclination matched usage: "Premier lawyer" got 40 times more hits than "premiere lawyer."
Now I should acknowledge the limitations of this. Sometimes one may consciously prefer the less common term. Sometimes one may want the term that is more common within a particular professional community, and not English usage generally; in particular, if you have free Lexis, you might want to search edited newspapers prose in preference to unedited Internet prose. Sometimes usage is split more evenly, so the results are less definite. (Perhaps the slightly more common term is seen by some, rightly or wrongly, as inferior, so one might want to go with the slightly less common term.) And sometimes the searches might be skewed by false positives (e.g., "Defendants were 'operatives' for B.C. Premier, lawyer argues").
But when one term is 40 times more common than the other, it's a pretty good bet that one should go with the more common term, unless one has a compelling reason to the contrary. In Horace's words, follow "the will of custom, in whose power is the decision and right and standard of language."