eh, i guess the answer would be yes if your not OC-ing but it would be really easy to get the E6300 up to E6400 speeds, basiclly safe too since 2.13ghz is still pretty low comparded to what some C2D's can be pushed to. in my opinion you should get the E6300, oc it then you can just pocket the $20 difference between the 2!
The conroes are excellent overclockers and though you might not want to do so today, you might be open to the possibility tomorrow.
The E6400 has an extra multiplier (8x266=2128 MHz vs 7x266=1862 MHz) so besides running faster than the stock E6300, it's a little more flexible as far as overclocking goes.
Either way, the E6400 is the better choice, given the small price difference.
The 6300 and 6400s are the low-end C2D processors... for now, they don't come any slower... however, even those are still very competent processors. I'd think of the 6600 and 6700 as the mid-range C2Ds.