The E4300 is a 1.8GHz that also lacks VT, the E6300 is a 1.86GHz part with all the trimmings.
A good thing about the E4300 is that it runs on a 200MHz FSB at stock, so you're less reliant on the motherboard and memory when overclocking. In fact, a 3.6GHz clock speed only requires a 400MHz FSB, which is pretty attainable on many motherboards out there and will run DDR2 800 on a 1:1 ratio. The E6300 is a better perfomer at stock but, for the most part, the difference in performance is negligible. I'd venture to say that the E4300 is the better buy but I still consider it overpriced (it should be a ~$130-140 part, not $170ish).