Kabuto wins this, without a doubt.
Kabuto has an edo army, which should be enough to keep Hashirama on his toes/distracted.
Flower tree world shouldn't even be a problem, edos will not be effected by the pollen and Kabuto should naturally be immune to it considering he worked with Orochimaru.
Wood style wouldn't help Hashirama against Kabuto, after all he's a slippery snake....
The fact that they're in a cave doesn't matter, Hashirama would destroy in anyways with Deep Forest Emergence or Flower Tree World.
Inorganic Reincarnation is also going to be helpful for Kabuto.
Hashirama has a lot of chakra though so he shouldn't worry about Kabuto taking it.
If Hashirama enters SM he could fight Kabuto from close range, but I think that Kabuto would win in a 1v1 either way.
You guys are also forgetting that Kabuto has a technique/poison from his snakes to stop Hashirama's wood style.
Overall this is a bad match-up for Hashirama, Hashirama specializes at long range fighting, while long range technique are useless against Kabuto, and Kabuto is also extremely fast and has reflexes that are easily on par with SM Madara's.....
Kabuto takes this high-diff on his own, mid/easy diff if he has numerous kages and the entire Akatsuki at his disposal........