Try and avoid a foil if you possibly can.
IMO if it needs a foil, there's something wrong with the boat/motor set-up.
I'm sure there are cases were it is unavoidable and it won't get on the plane without one, personally I don't like 'em.
have a look back thru the pages, there's been lots of debate covering the pro's and con's of foils and permatrims. think the general consensus was, if you have to have one, look at the permatrim.
regards