My vote is for solid mount. I say that based on my current conversion which has air ride on the front and back axle and rides very very smooth.
On my last trip I actually blew all of the air out of my chair for the first time and was amazed and the cool ride (floated along) the air suspension gave me.
With a boot mount you could keep your air cab, but if the boot ever had a issue, like wore out or cracked or just started leaking, that could give you real headaches trying to replace it.
Keep us posted as you move along with your conversion and please post pictures!
2012 Showhauler 28'6" Motorhome on a Columbia w/ 450 Mercedes.