LBTC - Changes in HR are an effect of overall efficiency of the heart to beat in such a way that it pumps more blood per beat. HR (including recovery time) is mostly effected by overall training and is not usually trained using intervals. Intervals are meant to train you body to deal with energy demands and produce power, HR change is a bi-product of that. So I think, and I may be wrong, that in terms of what happens physiologically, as long as you train with sufficient (that means at least a zone 1) your HR reco time will adapt for the better.

Having said that, my theory is (note I said theory, the following has no basis in any research I'm aware of or facts of any sort) that seeing as your HR is a muscle and any muscle can be trained and the best way to train the muscle is to get it to do the thing you want it to do.... I would vote for higher intensity, short duration with enough rest to recover into your Zone 1 heart rate in between.