I use almost exclusively slow film for pinhole because I often get more motion in my shots. What I mean by that is essentially the reasoning behind people using ND filters during daytime shots. I like Velvia because unlike when shooting something like my Mamiya, the reciprocity actually gains you a little latitude. If I'm off by a second or two, I haven't noticed thus far. With the vivid colors, the pinhole effects really stand out and make for an interesting image.

my $0.02