In my opinion buying products for particular hardware characteristics (6 cores vs 4) that don't translate into performance gains for your applications is letting the tail wag the dog.
To me, hardware characteristics are a means to an end: performance, power efficiency, durability, etc. Whether my computer has 1, 2, 6, or 50 cores, I don't really care that much. What matters to me is how well does it run the software I'm interested in. This is why I tend to prefer fewer faster cores over more slower cores. Most code isn't multi-threaded, and it will run at the speed of a single core and leave the rest idle.
Whether the performance improvement by going with a i5 over a AMD chip is worth the increased cost is your call. I can't really help you there. Intel i5-2500K chips will out-perform any AMD quad core by a large margin in running most software. Hell, it even out-performs AMD 6-core chips. If you look at this from a system perspective, the Intel chips give you more performance-per-dollar than the AMD chips. Right now, the only reason to go AMD is brand preference because quantitative measurements don't favor the Phenom chips, even when you factor in price.