maintance wise. the concrete floor may be the best. Never thought of the rubber flooring. I don't see why it wouldn't work. Every sauna I've been in though has the cedar slat pannel floor for the water to run through.
2 kinds of concrete , cracked and hasn't cracked yet They make that EPDM ( the rubber roofin you mentioned ) in different thickness' .060 is pretty heavy duty stuff , A big sheet with a floor drain ( don't use roofin tar near the rubber ) then use strips of the leftover rubber for "slip sheets " loose laid under the sections of your flooring sections , lets the wood move as it needs to without rubbing holes in that waterproof layer . They even make rigid insulation that is tapered in thickness to make sure things drain good.
Just my .02 ,But, If you know a comercial roofer , it may be worth looking into
Phil
�The public cannot be too curious concerning the characters of public men.� �Samuel Adams
"All men having power ought to be distrusted to a certain degree." --James Madison
) then use strips of the leftover rubber for "slip sheets " loose laid under the sections of your flooring sections , lets the wood move as it needs to without rubbing holes in that waterproof layer . They even make rigid insulation that is tapered in thickness to make sure things drain good. 
