Hi
I am in the lucky position of owning an Emax I and II, so I can give you a few pointers. There are a few differences in sound, but they are subtle. The Emax I has lovely SSM analogue filters, but they are unfortunately spoilt by the low bit depth of the controlling hardware which causes audible stepping when modulating them from a modwheel or some other MIDI source.
The Emax II filter is VERY close and despite being a digital filter, is lovely and warm sounding, but does not exhibit the same stepping that the Emax I has.
The Emax I has that 8-bit grit if you want that and can get closer to that EII sound.
For my money, the Emax II offers better value for money as it gives you more memory (plus options) and SCSI as standard. the sound is close enough, but you get more functionality and power. The Emax I has an undeniable character to it's sound (which is why I have one) but if I had to choose 1, then it would always be the Emax II.
hope this helps