I'm out of it for a little while and everyone starts getting delusions of grandeur...
I still don't get it myself. It seems so incredibly out of place to have him suddenly become this old man acting as if he were egging on a boxing match. It's hardly the time for being excited, in his role as Palpatine he should be deeply concerned about his own fate and that of the galaxy rather than having fun, and as Sidious he should be trying to keep as low a profile as possible while willing his favourite new apprentice to get rid of the old one. It just baffled me. I would argue it doesn't contrast Jedi particularly well either, though that's perhaps what Lucas was aiming for and I didn't think of that. The trouble is, he's pretty much acting the same in both situations, pleased at the fight and willing his new apprentice to kill the old one. It's just that in Jedi, despite having to make no effort to hide his glee, he's far more subtle.