On April 22, the price of the E6600 should drop from the current price of $308 to $235, which is $4 below that of the current price of the X2 6000+. So, if you are buying NOW, get the X2 6000+, but if you want to wait 2 weeks, get the E6600. Just don't buy the E6600 before the price drop!
I do not believe you would be able to tell the difference between the two chips at stock speeds; they are pretty much equal. The E6600 overclocks much better than the X2 6000+, but you probably don't want to get into that either way.