Except for key spells available only on one or the other when you have the traits that improve CON or THM abilities by 10 on other classes you can kind of mix and match to suit your play style with an eye to the fact that CON AOE functions somewhat differently if you are using them on THM. As long as your mindful of the differences where they exist it doesn't matter all that much at this point.

You can always leave the core spells the same and jump back and forth for those key abilities though depending on the situation at 50 from one big fight to the next. It's kind of fluid like that.