This topic came up before- possible on "the other board"'s site. I think the best answer I heard was that either would work but "P.E." was more traditional and "PE" more contemporary.
It also doubles as "plotter engineer" when I have to fix our archaic contraption. Our CAD guy is a great drafter, but not too swift when it comes to fixin' stuff.
Oh and, in he U.S., it is PE (or P.E. if you must). P.Eng. is used in Canada. There is no P.Engr. Though less common, some people use RE (registered engineer).