Oooh, tricky one.
Until recently, I think we'd ALL have said AMD by far.
I still remember when the first Athlons came out and totally destroyed everything Intel made for years, and the only way Intel could fight back was by forcing their P4's to run at higher clock speeds and higher voltages that they'd normally not run them at.
Heck, at the time, even the old K6 CPUs were slaughtering Intel's CPUs at everything except one thing - Floating Point performance, and that's what forced AMD to make 3DNow! (Because they hadn't found a way round Intel's FPU patents until the Athlons, which was basically a K6-3 with 3(?) of the new FPU cores)
Now things are not so clear.
Overall architecture, I still say AMD - The Hypertransport links are fantastic for multi-CPU stuff, and building the memory controller into the CPU was a smart move.
The Core2's *are* faster at this point (With 32-bit code) however, whereas AMD's DDR2/AM2 platform is kinda suffering. Until they figure out the why, they're gonna have problems competing with Intel.