In reply to cb_6: Id give both a go and see how they feel. In my experience it's usually pretty obvious when a certain exercise is hitting the spot.
In addition have a watch of this: If you're trying to train PE (or any other energy system) it's important to get the intensity and rest right, otherwise you risk slipping in to endurance/strength during your workout which is obviously not very good if you're aiming to improve your PE.
Jon, specific training can offer much better gains if you do it properly. To use PE as an example, if you're redpointing routes you'd be lucky to get 5 or 6 good redpoint burns in a 2hr session, and within those redpoint burns you're looking at maybe 2-3 mins max where you're really getting boxed and testing your PE. So over a 2hr period you're getting maybe 15 mins of proper PE training in (bear in mind that this assumes you do everything else right, like choosing a route of exactly the right difficulty, resting properly etc. so that no redpoint is wasted.)
Compare that to 4x4s, where in an hour you can easily get 30 mins of soul destroying PE training done, and any mistakes are easily corrected. In the long term this higher intensity will ead to better results.
That's not to say redpointing isn't useful, just that it isn't necessarily the best way to improve your PE if you've already got some redpointing skills.