I've flown the Six, 210 and my A36.
For sheer volume, the Six wins - but it's like flying an old school bus. It's a lumbering beast. Not fast, but very stable. The fixed gear version is SLOW.
For speed, the 210 and the A36 are very close. Things like engine upgrades and turbo choices will be deciding factors when comparing the Cessna and the Beech for speed. In the end, they are very close.
The A36 will burn less fuel than the 210 and it's MUCH nicer to hand fly. Controls are crisp on the Beech.
Cabin space = as I said above, the Piper wins here overall. The Cessna feels a bit wider up front than the Beech, but the last row of seats on the Cessna is for little people. You can sit an adult in the last row of an A36.
The Cessna has two doors up front, The Beech and the Piper have one door up front.
Both the Beech and more modern versions of the Six have club seating with nice wide barn doors for passenger entry.
If you care about fit and finish, the Beech wins - hands down. The Beech is also built to Utility standards whereas the other two are Normal category.
For me, the A36 is the absolute best choice. My useful load is around 1300 Lbs. If I want to pay for the paper STC, I could increase that to over 1700. If I needed more cargo volume (actual space, not weight), I'd consider a Saratoga.
Join Beechtalk.com - It's free to join. There is a WEALTH of knowledge available. You can easily find an owner who's listed as a Mentor in your area. They will be happy to meet with you and show you their plane. I've done it for people in SoCal. Always happy to go flying with someone who wants to see what an A36 is like.