I must say that when both of these models were unveiled, i was pretty impressed.
The W203 was a breath of fresh air, going away from the boxy design of the W201 and W202 and following the trend of the W210 (first E-Class with "bug eyes") and the first generation CLK (C208 is it??).
But now the W204 has gone back IMO to the tradition Benz design of yesteryear and they've done a great job. I think the car is good looking from every angle and it's bang up to date.
I would prefer the W204, not just because its the new model but because I think the improved quality

of the interior and reliablility (the rigorous testing during development) should make it a better car than the W203.