Get the V8. I used to have the 535i and now the 550i. The F10 550i with adaptive drive feels more planted and you don't feel the extra weight at all. Now, you do feel the extra weight on the M5, surprisingly. The V8 is way more enjoyable to drive.
My experience with the 550 was quite the opposite. It felt nose heavy. The transmission didn't shift as smoothly and easily (2nd to 3rd was particularly "clunky"). But it sure did accelerate with ease. That was what I liked. Mine was a 2013 with AD.
But I was so disillusioned by my 550 because of these and other problems (throttle tip in and turbo lag, for example) that I revolted and swapped it for an i3 the moment I got the lease pull ahead offer. An i3 for crying out loud.
This time I'm going back to the 540. I was considering the M5 hoping it wouldn't have the same problems but now that you mention it too had a weight feel problem, well, bummer.
I would wait to see how the different variants of the G30 drive and feel, but I want out of the i3 ASAP. Don't get me wrong, I love it. It just doesn't have the right vibe/feel to it. I want back in a sporty luxo cruiser. And I don't want to risk the same problems again with the 550.