Superior how? The E8400 is clearly faster but it's also what, double the price? The athlon 2 has a much better platform (mobo upgradability) but if you want raw performance and dont care about that then the E8400 is the best option.
Any particular reason why you chose to compare those two cpus? Some details might help you get a better answer.
Inside the AMD Athlon II processor is an architecture largely based on AMD's Phenom II 'Deneb', but optimized for dual-core processing. Instead of a large 6MB L3 cache that's shared between four cores, AMD has stripped out the L3 altogether and implemented a more sensible 2MB of L2 cache. Like the Phenom II, the L2 cache is split into a single megabyte of per core.
Sticking to a smaller cache size reduces overall cache latency, while increasing the size of a cache means greater hit rates, which reduce the amount of trips needed to fetch information from system memory. Having a large cache makes sense for quad-core processors, but for a dual-core processor like the Athlon II it would most likely be under utilized. Large caches also makes the CPU's silicon die larger, which increases the amount of power consumed and heat produced.
AMD has kept the size of the Athlon II processor small, about 117.5mm2. This is in part due to smaller cache sizes, but also thanks to moving to a 45nm manufacturing process. This makes the Athlon II X2 250 one of the smallest processors AMD has ever produced.
Yeah the price for performance is weird for the Intel dual cores.... I have no idea how Intel is still keeping them at such high prices. One thing I can vouch for is I have an overclocked E8500 and this baby has never let me down on anything and I mostly game.