My advice is to look at all the characters you want to get EX+ for on the
Opera Omnia Database and see how all their buffs and debuffs interact. Do look up how they are in the JP version of the game, as that is a good way to check their long-term viability.
Both players and enemies can only have six de/buffs at one time and buffs of the same type do not stack ( Max BRV II overwrites Max BRV I, etc.). This puts a bit of a limit on how much you can buff characters or debuff enemies and can help you figure out which characters are a bit redundant if they are in the same party. By this point in the game, most characters have at least one "framed" buff that is specific to them, so you'll usually want to aim for about four-five "generic" buffs so that you don't overwrite buffs. The same thing can apply to debuffs as a lot of bosses have "framed" buffs of their own that can erase your debuffs if you have a lot of them... or if you have too many characters that have the same kind of debuff.
The other huge thing to keep in mind for EX+ is that you can only use each sphere
once. After you use it on a character, that's it. To get another sphere of the same kind, you'd have to refine a whole nother Ex+ weapon from scratch. And that's... a lot of materials. So in a big way, the EX+ era is the big "crunch" time for people's rosters. Now's the time to figure out which characters you like using the most (or will like using the most if they aren't out yet) and stick with them.
Another thing I've noticed with Spheres (at least for the Jamming (E) ones) is that the debuffs on the target that the Spheres give are all generic debuffs. So the odds of them overwriting other generic debuffs of the same kind is very high. This is most likely an issue for characters that already give generic debuffs of the same kind of Sphere Debuff then get. Tidus would be a good example of this. His Slash Combo gives Speed Down already, but so does his Sphere. So his Sphere is probably best used on a character with no natural Speed Down Debuff. I'm very interested to see how the Sphere Buffs work, because if they're also Generic Buffs, then giving them to people is going to really come down to what buffers you usually work with and what buffs those characters already give out.
I want to fully purple her for sure when I get the chance but otherwise I have been like 
However I read Vanille EX+ next month and I definitely want to do her too, Serah as well. I feel like I should commit and EX+ this month as well just so I can maybe clear some chaos stuff but I don't know who. I got Duece's EX and I like it so her maybe but Yshtola is later this month ... there's also Rem, or Agrias maybe 
I managed to clear my first Chaos fight with the Machina Chaos, Ysh/Rem/Agrias, used 100 gems for the clear but I am still quite happy, Chaos doesn't seem quite so insurmountable anymore, might try again sometime replacing Rem or Agrias with Ultimecia or Lilisette with a Kuja friend perhaps 
