It's all about audio.
If you want to be able to bitstream the audio to an HDMI equipped amp then get the EP35.
If you need to use the 5.1 analogues on the EP35 to send TrueHD etc., to an old amp that can not decode the new codecs, then get the EP35 and connect it to your old amps 5.1 analogue input.
If you aren't bothered about any of this and are happy with DD5.1 over the optical link to your amp, then you might want to get the cheaper EP30. It doesn't have the 5.1 analogues or bitstream over HDMI.
My guess is that they are both the same in terms of picture performance, they certainly use the same chip for up-scaling DVD. I have just bought the EP35 because I want to bitstream audio (and having analogue connections is a bonus), otherwise I would have probably stuck with the EP30.