CLS diesel - 320, 350 or 350 grand?

[village idiot]

toying with getting one of these as opposed to a beemer 6-series (the cls is infinitely more practical)

I see that circa £12.5k will get me a nice low mileage 320 diesel and £16k will get me a low mileage 350 grand...

is the grand worth the additional £3.5k?