Tough one. I switched to a V8 E500 S211 in 2015 for its luggage and bike carrying capabilities. And I've just bought a V8 W221 to replace it because I love the luxury ride of the W221 ( always grin when the S class turns up as an Airport transfer) and I only ever use the tailgate to load bikes half a dozen trips a year.
Doesn't the issue comes down to what will you ACTUALLY benefit from?
The E63 Estate has street cred for sure, but will you ever use the power? I live in roughly the same area as you, and I have never felt that I needed anywhere near 500bhp.
I used to have a new E55, back in the day, and I always felt that I had to drive it on tippytoes - so much so that it didn't feel like driving - and this was the E55 not the E55K. (Although TBH, back then most of my driving was in Central London - not any more)
Whereas the W221 delivers luxury and quality in spades. If you cope with the jokes about the Chauffeur's hat?
Does size matter? It depends where you're parking it. Does it fit in your garage? If it's on the street in London everyday, then maybe. Elsewhere: not so much? An S500L W221 is fine in modern multi-storey's, thanks to the popularity of SUV's, they've just got bigger.
The magazines, like Car and Driver, will argue that the S212 is a better "driver's car" than the W221, but is your taste and driving, the same as a young Journo who still has car posters on his bedroom wall?
Another question is how can you consider an E63 against an S350L ? Shouldn't you be comparing engines which are a bit closer together?
No disrespect to any choice - it's just a matter of mapping the priorities, and actually driving the things in real life, on roads you actually use, to do things you actually want to do.
Just some thoughts from someone who's just been through the same choice process.
