The later 520d's (and 525d's) are 4 cyl where originally they were launched as 6cyls. They're more efficient at 4's but less 'nice'.
I do like the 5 series, esp the estate, but its a bit too big for city use and its approaching end of life now as its been around quite a while. As soon as I drove the Merc, I knew it was just the right fit. Not too large or small, nice to drive, low seating so you don't feel like you're driving a boat etc.
There are quite a few early build horror stories though, wind noise on seals etc, so I hope mine's improved or I'll be adding Merc to my list of manufacturers that wish they'd never heard of me.