Definitely go for the silicone Diva over the gum rubber Keeper. But if you can get the silicone Moon Cup manufactured by The Keeper for the same price, I'd choose that over a Diva. I've seen more leaking and not-unfolding-during-insertion issues regarding the stiffer silicone of the Diva, both here on MDC and the Yahoo! Diva/Keeper/Mooncup users group.
I love my Moon Cup (by The Keeper, not the UK one). I've only used it for three cycles so far, but I can't imagine going back to pads or tampons! Have yet to have a leak, and I wear it day & night - cd1 & 2 are my heaviest and I can usually go 6-8 hours between emptying it, and then every 12 hours for the rest of the days.