I had the chance to see Jeff play at a small club in L.A. (the Baked Potato) in '87. I was still in my formative years as a musician and all I can remember is coming away thinking that I have never seen another drummer with such incredible control of their instrument. It was simply mind-blowing to see these guys play up close. I'm not sure I would feel exactly the same way today, having been exposed to a lot more music since then, but it was pretty humbling at the time.
Stewart is in a whole different musical realm for me, so I don't even think of them having anything in common other than they are both drummers. I think of Jeff Porcaro in the same way as I think of Vinnie Colaiuta, but even a bit more "studio-ish".
Edit: (now that I've actually read this thread....I agree with much of what daweasel has already said)