The best thing to do is to pop along to a local dealer and give them a try. BUt then again, if you know you definitely want some Grado's, there's less than £20 between the two - I'd just buy the more expensive ones. And that's me recommending from a personal point of view, not as a retailer.
I have the 60s and like them (haven't tried 80s). Agree with FH though - if the difference is £20, and that isn't life or death for you, take the more expensive ones.
something to consider for those using Grados with iPods (from Cnet):
The SR80 is a dead ringer for Grado's $69 SR60, but the pricier model uses a slightly higher-quality cable and larger, deeper ear cushions. That might not seem like much of a difference, but the resulting sonic improvements aren't subtle--the SR80 provides better bass and greater overall detail. Those qualities, as well as the headphones' more natural midrange and treble, were readily apparent on our home system. We have reversed preferences on the road; the SR60's more laid-back tonal balance seems to bring out the best in pint-size portable players, such as our Apple iPod.