Senpai is correct modern Hepburn romanization and what you would see in any modern textbooks.
Sempai, however, is not really incorrect (as long as you are consistent). Like for example, the newspaper "Asahi Shimbun" still romanizes its name using an "m"
(ref:
http://www.asahi.com/english/ ) because it is grandfathered in from the older style. Even though most people would write "shinbun" for the romanization of newspaper, all the old names are still done in the old romanization style.
(the old rule was that you would use an "m" instead of an "n" if the "ん" was followed by "ば" or "ぱ")