Real dice, no contest. I like dice, I have quite the collection. I like the shapes, the colours, the forms. I also like the physicality of rolling the dice, that moment when they leave your hand but have not come to a rest -- everything is still possible during that time. The shared triumph when the other players can immediately see that you rolled a critical, or the groans when you roll a fumble... Nothing else but physical dice can give you that.
Of course, when playing online, that doesn't really work. For those cases, we use Dicestream through Google Hangouts. It's pretty neat, and it allows for the shared experience.
I've played diceless games -- well, actually only one, the Amber Diceless RPG and it's direct descendant Lords of Gossamer and Shadow. Of course there are no dice to roll in such systems. But I don't miss it there, because the system is designed with a much higher power level which gives such a different experience than the usual D&D-like thing.