It has been a very long time since I actually worked with new hardware, but I will try to answer.
Intel and AMD appear to have conflicting philosophies. Ultimately, the optimal computer would have a CPU on each RAM chip.
Intel go to great lengths to seperate the CPU from RAM and every other component. They have on more than one occation chopped L2 cache (type of memory) off the CPU to reduce cost and at the same time emphasise MHz. The least amount of cache ever used in an Intel machine is 0kb. Intel do currently stick a fair amount of cache on their processors so the above is only a generalised pattern, not a strict rule.
AMD invest in high performance cache. Afaik, the least amount of L2 cache ever used in an AMD machine is 64Kb though it is normally much higher. They also have some neat tricks to make better use of the cache.
Difference in cache performance is one of the reasons why AMD generally perform better at the same frequency. On the other hand, Intel generally provide higher frequency processors. AMD chips are sold by Performance Rating which is not indicative of the operating frequency - only Intel emphasise raw GHz.
Computers benefit from having lots of cache. So if I were to buy a CPU today, I would list all those with large cache and then pick the fastest of those which is within the budget.
There are other differences like the intructions sets. AMD 3DNow vs Intel SIMD etc. They don't make much difference, except that the Operating System needs to support SIMD before other drivers/software can. The same is not true for 3DNow. I don't know which performs better but they do a similar job.
If you are looking to buy, please don't take what I have said as gospel. Instead, search Google for upto date reviews of the chips. My knowledge is a little outdated
