First of all, the intinery is usually the thing we look for the most, and then price. As for the 2 lines, everyone has a different opinion and yes, most mass marketed lines are more alike than different, but I would take Princess over RCI. I happen to think the food is better for starters. I have read your listing of ports, both look like good choices.
ncl, carnival, HAL, Costa, RCI, commodore,chamdres,celebrity,Princess