While I am a long time Princess cruiser and usually recommend Princess, it sounds like you are happy with RCCL and therefore should probably stay with them; you know, "if it ain't broke, don't fix it". My Princess buddies are probably thinking I"ve lost it, but I am sincere.
You will definitely have an older crowd on a 12 day Mediterranean cruise on Princess. And beside the "Movies Under the Stars", Princess dosen't have as much to offer mid-teens as RCCL.
As for price, I don't think Princess has less quality; they just have a LOT of cabins to fill and price is one way to do it.
Whichever way you go, you will have a wonderful time. They are both great lines and both magnificent ships.