In northern Minnesota, if the sauna does not reach 190 or 200 F, it's considered wimpy. Get good pipes, insulated, double-wall. Observe all manufacturers specifications regarding placement of stove near combustible surfaces. Benches should be placed so that no person is sitting closer to the stove than the minimum distance allowed from the stove to the wall. Although there are arguments that wood can splinter, can harbor bacteria, can rot, it is very very comfortable.
Wood attains a more comfortable sitting temperature more quickly than tile.
When done, a wood bench can be wiped down with first a damp, then a dry cloth, while crud will collect in the joints of the tile. Wood is also cheaper than tile. If the head of a screw or nail must be exposed to a seating surface, recess the head. The most comfortable floor also is wood planks, with crevices between the planks to allow for swelling and also to allow for water to escape to the surface below. Concrete is nasty and cold, and requires a drain.