Both MS users, and both skilled ones.
He's a fairly good counter for Uchiha in general, with Genjutsu protection ( Hachibi ), Samehada for Susano'o and his Substitution tentacle for Amaterasu? ( And Samehada I guess )
That's why Itachi's simple Genjutsu:Sharingan worked on Bee, hm? If Itachis intention has been to kill Bee at that time he would be dead already.
Read chapter 549 and you will see- Gyūki has to realize the Genjutsu first of all to counter it.
Samehada can't absorb Amaterasu, because its even hotter than usual Katon. At the beginning of chapter 549 Samehada made a painful experience because of Itachis Katon.
Furthermore if Amaterasu would hit Bee(not transformed), it's over. Kawarimi no Jutsu woudn't work under those conditions. On the other hand it's due to Sasuke Bee could escape. If I remember correctly Gyūki was in heavy pain and coudn't do anything. At that time Sasuke segmented Gyūkis tail to save Karin, right? Bee used that possibility for Kawarimi no Jutsu.
BT:
Samehada can absorb chakra by contact-with restrictions-. Therefore it should also work on Susanoo. However, it seems Samehadas ability is depending whether the sword likes that kind of chakra or not.
"My blade seems to have taken quite a liking to your octopus-flavored chakra. It's rare that Samehada gets this worked up..."