Tonearm mount to the plinth vs arm board vs rotating arm board vs isolated tower

For the reason atmasphere just stated, I elected to have an armboard for my VPI HW-19 made from the same material at the top plate of the floating subchassis: acrylic. Delrin is harder and better damped than acrylic, but an armboard made of it will vibrate differently that does the acrylic plate, setting up another division between arm and main bearing/platter, and therefore between cartridge and LP (where the road meets the rubber ;-). Both the acrylic armboard and subchassis top plate are very firmly secure to the stainless steel bottom plate, minimizing relative movement between the two.  

When I was building slate plinths for various of my turntables, I adopted the practice of not creating any tonearm mounting boards or accommodating for them.  I therefore had to limit myself to surface mount tonearms, like Chakster says. These included the Triplanar, Reed, Dynavector DV505 and maybe a few more that I don't own.  I bolt the tonearm directly to the plinth.  Nothing moves.  I am not saying this is the best way to go; it certainly is not the most convenient nor the most flexible, but I think it adheres best to the principle of coupling the tonearm pivot to the platter and bearing.  For highest flexibility to use any arm any time, an outboard platform would seem best, if you're being pragmatic above all else.  A very heavy outboard arm pod that sits on the same support structure as the plinth itself is probably an acceptable compromise as far as coupling.

I think that it was Einstein that said  “Does the Station stop at this Train”

Now I know that I’m stretching the point a little but as Atmasphere said, to paraphrase..Relative movement between the arm mount and the platter is a bad thing.  We can have them both move in sync, within reason. 
If you agree with this principle, the arm mounting design options are narrowed considerably.

Thanks for all the information.
 I am considering a direct mount tonearm.
is there a sonic difference between Reed 3p and 2a

Yes.