Not often when shooting unless you're always filling the buffer up with long bursts. And I suppose the particular body has a lot to do with that anyway. And as I use Canon and really know pretty much nothing about current Nikon bodies you can largely discount my opinion there.
Transferring to the computer, then yes, I'd say the extreme III is faster. But we're not talking huge differences. Bottom line... I think the Extreme III is worth the difference (for me) The Ultra II is quite good at a lower price. You're unlikely to be disappointed with either.