So we've had the opportunity to do both with both kids. DD was turned into a princess, DS a knight at BBB. This was expensive but fun for both of them, and the photos at Castle Couture afterward are some of my favorites of them together when they were littler.
We did the Pirate's League later that same trip and both kids were transformed into pirates (DS into Jake from Jake and the Neverland Pirates, and he looked amazing). That was far more affordable - and a much better deal as we went through the Pirates of the Caribbean gift shop after and added up what they got vs the cost of the session - and both kids reported they enjoyed it more than BBB. My DD was a bit older at the time - 9 - and DS was 5. They do some pretty incredible photos in the map room after the makeover and those pictures are amazing too, for different reasons. And if you time it well, you can have them attend the Pirate's Tutorial after, which is just across the way with Jack Sparrow. One of the most fun things we've done on a WDW vacation.
Whether it's worth it to travel "all the way to Disney Springs" depends on how much your DD wants to do this (or whether she is aware of it). Personally I am not sure I would go to DS just for that - but if I could work it into other time we would spend there/ADR there then that would be worth it to me. I would otherwise focus on getting a time at MK.
In our family we try to do "all things equal" so if we do for one we do for the other - so if it were me and I couldn't score a BBB ressie for my DD I would cancel the Pirate's League for my son. If you are visiting MK more than once or have hoppers, perhaps one day can be her special day (whenever you can get BBB) and the other can be his for Pirate's League.
Let me know what other questions you have about each of the sessions. Both are really special experiences!