Sage Mode is essentially the same for every capable user. The only difference is the user themself.
For example; Jiraiya had to have the toads ride on his shoulders to gather sage chakra. Naruto didn't have to. Both were able to utilize Sage Mode, but Naruto could do it more efficiently.
Now, back to the matter at hand (concerning Kabuto and Mitsuki).
In my mind, Kabuto is still definitely the strongest user. He was the strongest character in the war arc until Kishi pulled the terrible ass-pull called Izanami.
He had Jirobo's, Tayuya's, Kidomaru's, Kimimaro's, Orochimaru's, Juugo's, and Karin's DNA, and he still has their DNA.
Unless Mitsuki also has similar DNA, he won't be close to Kabuto's power.
Still, Mitsuki > Next Gen