It's all about marketing, same car but have different levels of luxury.

The E300 is aimed at the primary producer, who wants a benz but doesn't want all the luxury. He can fill it up on the farm, jump in with his muddy boots and so on. It also depends on options, most E300's I see are usually standard with limited options.

The E320 on the other hand, is aimed at the executive who wants all the gadgets and electrically operated equipment, most E320's are highly optioned.

So it all depends on what your needs are, if you want a very reliable, low cost to run and maintain and reasonably luxurious, then the E300 is it.

Or if you want more luxury and creature comforts, costs more to run and maintain and is very expensive to repair when electronics start to fail, then the E320 is it.

I hope i'm not putting you off, but it all depends on what your primary needs are.
ozzy