Box speed. These (and to an even greater extent Velvia) are sort of exceptions to underexposing slide film slightly. Both already have saturation to spare, which is the reason people used to underexpose slide film, and both (particularly Velvia) are quite contrasty. Box speed is ideal but with these, if I had to err with these, I'd err on the side of 1/3 stop MORE exposure, as that would reduce the contrast slightly. But slide film in general and highly saturated and contasty films in particular have little latitude. Carefully metered box speed is best.

FWIW, I personally always preferred E100VS to Velvia as being not quite as hard to deal with in contrast while still having really high saturation.