You talked about doing a certain amount of kms, but what I don't know is if you are doing only once or every weekend.
Neither do I the reasons why you are doing such a rather distance. You'd better first know your aims and then you will can plan properly both your kms and intensity of your training.
Anyway, as regards to do one of the two kinds of training you propose to do 600 kms, I woud choose one of them without doubt. The best the one you mantain a higher cadence, because if you can stand it during the whole training, you will find your legs a bit less tired, and, therefore, you will can recovery better from such a three days in a row. On the other hand, I recommend you to keep a steady heart rate you know you will hold during the whole training, as you become tired sooner if you often make change paces.
I hope this post can help you, and if not, I will be pleased to comment all the aspects you want to talk through.
Greetings from Spain,