Both will do 3 GHz, but the E4500 has an 11x multi, and will do 3.4 GHz without getting unreasonable with the voltage (and probably higher depending on the board and the cooler). I've had an E4500 with a Arctic Cooling Freezer 7 Pro that I was able to overclock to 3.4 GHz while keeping the chip at low-mid 60s Celsius. This was on the DFI Blood Iron P35-T2RL, really nice results for a board of this price, it will do up to around 550 FSB with the proper settings.
Anyway, the E4500 is a newer stepping, M0, and it's pretty flexible. E6300's need a higher FSB to make it to the same speed, which also means better memory, etc etc. I can't think of any advantages to using an E6300.