I doubt anybody has seen enough of these two to make an informed choice. Granat is one big dude, but Miller is more proven at the moment. On the balance of the sp**** evidence, I pick Miller.
Just watched a few of Granat's fights. Hard to tell a lot because of the calibre of competition. He took a couple shots from Darnell Wilson well in their fight but mostly just teed off on the old Ding-a-ling Wison. The Swede looks pretty fluid for a big guy. This would be a good matchup for both of these guys. Supposedly Granat sparred well with both Povetkin and Klitschko. He also called out Dillian Whyte. I think this would be a fairly close fight on paper. Might give a slight edge to Miller though.