Both are more or less the same. But based on a few benchmarks that I've seen (there aren't that many for the E6300 or E6400) at worse the Conroe E6300 performs just as well as the X2 4200+. In the best case scenario Conroe performs a bit better.
That is true for both applications and games when both CPUs are being overclocked. Personally, I would choose Conroe, but I going for the E6600.
Click on the following link for a list of reviews:http://forumz.tomshardware.com/hardware/Post-NDA-Conroe-Benchmarks-ftopict191416.html