Dear Pammi,
I have had an E-96 for about a year, and have used it extensively (though only personally; not in a gig situation). It functional capabilities are on par with the G-600. In fact, most all of the controls are identical to the G-800 arranger keyboard. But it is based on a previous version of the SoundCanvas series, so the voicings are limited to 241 voices; rather than the 689 of the next generation.
My local dealer has shown me the E-300, and I did get to play on it a bit. Certainly, it benefits from a newer generation of sounds. Along with the E-500 (which I've also tried), it is a simpler keyboard to work. Personally, I'm thrilled with my E-96, because of all the options it allows me to use in modifying my styles and arrangements. So, if you are looking for quality of sounds and simplicity, I'd suggest going for the E-300. If you like to dabble and explore the capabilities of the keyboard (and are not limited by 28 note polyphony), you may want to consider the E-96, if you are getting a good price on it!
You may write if you have specific questions.
Good luck in your decision.
Del.