A 4200+ stock performs close to an E6300. They both are around $180-$200 right now. But this is what I believe to be the deciding factor: Are you building from scratch or do you have a s939 mobo? I say if you already have a s939 mobo I would just get a X2. You can get a 3800+ for 180, a 4200+ for 200, or a 4400+ for 260.
If your upgrading from s939 then you would have to get new RAM too because Conroe uses DDR2.
But if your building from scratch I say go Conroe.