The difference is Tulip is of no use and there's unlimited supply, right? Even if bitcoin is a bubble, it won't happen so like tulip IMO.
As soon as they begin to say that there is a future for crypto-currencies, this will be a signal that it's time to sell.
I feel the same too, but I still can't figure out why though. Maybe that's from what I've learned which says 'when everybody is buying, start selling' or something like that.