Start with why you want tubes. Then think about how much you can spend. Personally I believe that tubes are for acoustic music. I see no reason why anyone would benefit from reproducing heavy metal at 95 db with tubes. That is SS territory. Tubes are for sensual, subtle sounds.
If you still think you want tubes, or if you now see bi-amping with SS works better for you, the rules are the same. Two chassis cost more than one. Monos enable you to shorten speaker runs. Monos, even if they share a chassis will have separate power supplies allowing for better separated channels. Then there are layout implications, bragging rights and forum topics.
Remember, though, that bi-amping is not a goal but a means to reaching one's goal. Unless you need to do it, don't bother.