why not pop to your big local second hand car dealer? go take a look at a load of E46s and E39s and see what you think of the boot?

dont forget aswell it's not ALL about boot capacity, you need to consider ease of loading. biggest problem i had on my E36 saloon was that even tho the boot was quite deep, nothing would fit through the bloody opening! and yuo have to lift everything over a few inches of "lip". i love the fact with the e39 touring the boot opening is just a big gaping hole and you can just slide in whatever you need without lifting in in through a saloon shaped boot opening.