Mark,
I've thrown all three. I would agree with the versatility comments here and throughout the Roto threads. But the one word that describes the difference between the Cell and let's say the Attitude Shift is continuation. Storm has clearly gotten the message with the Cell - they are now offering a 'comparable' (note to Roto People - I said comparable so don't bite my head off
) ball with the new Dimension. Again, continuous seems to be the word.
If you are looking for a med heavy to heavy oil ball get a Cell or a Dimension. Cell has a great track record and the Dimension seems to be the Cell's slightly longer, slightly snappier cousin. Pick your ball motion and pick up one of these.
If you want to see what a tweener can do with a Cell take a look at the PBA Summer series events. Danny Wiseman is a perfect example of what the Cell's continuation can do for your carry.