If it is a quick trip to Kazan without either the need or desire to transit through Moscow and if cost is not the issue (presumably OS will be more expensive overall), then I would say go with OS via Vienna. They probably don't fly daily though. Agree with chalf that S7 is all right, especially if it is a B737. I've taken the sleeper train which probably costs about the same as flying...unless you are into trains, I would fly. A foreigner not used to travel by rail in Russia will not be very comfortable - again, unless you want to do it for experience.