I think indoor hard, because they have no permanent indoor claycourt and last year, they said after the tie against the USA that it was tough to make a proper one for one occasion because of the coldness in Switzerland at that time of year.
Moreover, this year, the tie will be the week after the Australian open and not one week later, and Fed will probably prefer playing on a indoor hardcourt at this moment.
But I think they would choose a slow one, as it's better for Wawrinka and worse for Stepanek.
Wawrinka often gives problems to Berdych by the way. He leads the H2H 5-4 (3-4 on hardcourts, 0-3 indoors)
Do you think there's any chance they could play on clay? It's Wawrinka's favourite surface and it would almost certainly mean Stepanek won't get a win. I would arguably fancy Federer a lot more on clay vs Berdych than on a hard court too.
I have to say you are extremely well informed and I enjoy reading your posts.