I just whipped up a quick list of the professions in use in my upcoming RMSS (RMFRP) game, along with the RM2 equivalents, and a ranking from 1 - 5, where 1 = significantly different and 5 = pretty much identical.
https://docs.google.com/spreadsheets/d/1WKE7TsX6Ur0ENVn4huY1TIc8sSy9QavrRUDEeXoOL90/edit?usp=sharing
The link is fully editable, so anyone else is welcome to add in more equivalencies. I only have Companions I, II and III in the RM2 line, so there are plenty of old professions I was never exposed to.
In general, RMSS professions tend to be better balanced, especially with respect to spell lists for the optional professions.
The RMSS Priest profession (from Channelling Companion) can be used to create a good number of the optional Channeling Professions from RM2.
Despite the fact that the systems are more similar than they are different, I don't think you'll get much use out of the RM2 Companions in an RMSS game. The points of difference are significant enough that it would take quite a bit of work to make them compatible with RMSS, especially when it comes to professions.
Personally, I prefer RMSS to RM2, but with a lot of skills removed and some adjustments to skill categories (merging quite a few of them).