I have the Salem and the Jean Bart. Both can be fun ships. Haven't played all that many rounds but here are my few cents so far
Salem trades better DM radar range for faster reload/British heal, and has the potential to punish your general pubbies if they arn't careful.
Can't say its better than the DM. it's just pleasantly different. However, her short radar range prevents her from being in the top picks for clan battles and such.
Jean Bart has everything you would expect from a french BB. Boosters make you highly flexible and the reload consumable allows you to pump extra rounds into broadsiders. that being said, her shells can sometimes feela bit lackluster to me but the dispersion and handling are top notch. If you liked the Richelieu (ALOT) and/or if you are very surgical with a good set of guns, then this is her up-tiered relative on a bit of roids which you'll find right at home with.
Also have i mentioned rather nice AA?
Musashi on the other hand i have trouble recommending. for a prem ship it doesn't earn much for its tier, very lackluster secondaries - nonexistant AA ( D: ) and is overly dependent on MM to have a jolly good time. I see that you already own the Yamato. Fantastic. I trust that you have been snickering along at the Musashi's who the MM didnt't quite agree with while still having to fight you, the much more superior sister ship >: D
But i guess if you're into collecting ships - Interested in the Musashi herself in that regard - then go for it. if she lands on top of a team - Jolly good time like i said.