Until recently I had a W211 E55 AMG & it was a great car.
It was very stereo typically Mercedes and all the good & bad that brings, well built, very refined in my case huge torque, but hilarious handling & expensive parts & servicing costs.
I picked this over the equivalent M5, RS6 estates because it was cheaper to buy, more reliable/less weak points & also significantly bigger load carrying.
I can't remember the exact numbers but I seem to recall that the Merc & Volvo are 40-50% bigger load carrying space than the 5 series & A6.
However, based on what you've said I would choose a 530i.
I had one of these for a month back in 2009 & remember thinking it was the perfect mix of everything, fast yet still 30mpg, fun yet still refined, enough space, etc....