Im writing this review basically because all the other reviews on ebay are now out of date. Uptil about a month ago both companies offerings had advantages, although apart from multitasking, the AMD processors were generally better especially for gaming.
However Intel has now brought out its "Core 2 duo" processors (codenamed Conroe and Allendale), and to put it simply, even the budget versions of the Core 2 duo beat AMD's best processors. Especially if you are willing to overclock them, and even if not. In my opinion you should NOT buy a non-core 2 computer or processor, no matter what, unless you are severely budget restricted. (the E6300 which is the cheapest sells for as little as £150 on ebay)
It can however be hard to find computers using the Core 2 processors at the moment, so basically wait a few weeks, unless you have to get one now. (Ive been waiting since may for these to come out).
If you have a really tight budget this is good news still, because AMD have slashed their prices, in an attempt to combat the Core 2s, so a budget Sempron is now incredibly good value compareed to a few weeks ago, and you'll find you can get a lot more for you money than you were previously able to.