Both are good, and for different reasons. At the bottom end of the range, it's really a coin toss -- a basic single or double core processor is a basic single or double core processor. As you move up the range, though, the strategies for more power make themselves clear. Intel tends towards fewer faster cores and better pipelining; AMD goes for more cores and increased parallelism, and has been lagging Intel in litho screen resolution for the last little while.
If you are using applications that aren't highly multithreaded, and work mostly in one application at a time, then you'll find better performance with fewer faster cores. If you or your applications are heavy multitaskers, then the performance of the system as a whole is more important than the performance of a single core on a single task -- all of the individual tasks will feel more performant because they don't have to queue for processor resources as much. Another way of looking at it: eight cores is only better than four if you're using more than four of the cores, but if you are then you can see significant gains.
But that only applies within a certain spectrum of parallel computing -- a lot of the really massively parallelizable non-graphics tasks these days, especially in scientific computing, are being offloaded to the GPU, which makes both Intel and AMD CPUs look like they're standing still. And that's the way things are this week -- who knows what tomorrow might look like?
That used to be true, but it's the other way around now. The hot gaming rig right now is the Intel i7 2600K overclocked until it glows, while the AMD FX-series 8-core processor is the king of megatasking at the moment. You blink, and everything changes.