Hi Jadarite,
There is a long, long thread in the forums on this already, and views range from "both are equally acceptable" to "only the most uninformed ignoramus would use the pronunciation with the T." (To counter that latter point, one of the posts pointed out that it was only when the masses started learning to read and saw that there was a T in the word that the T started being pronounced - a sign of literacy, actually, and not illiteracy. But anyway...)
I say "offin" but I would certainly know what word you meant if you said oft-in.