I would have to cycle pretty fast to burn the same calories as running or just do it for a lot longer. There are not many places where I could safely ride fast. It is less hard on the joints though. I think the best exercise is the one you want to do. I am happy enough running and it is convenient to get a decent workout in the time I have available.
That said, I would like to cycle more, but I have hardly done it recently. I do need to sort the bike out as the back wheel is bent out of shape. That is something to sort out over winter.
!BEER
yes, agreed. I do like going fast on a bike but the temptation to step down a gear when you start to have a bit of difficulty would always be there as well. I suppose it would be a better exercise for when you can't run because of joints and what not