I also have an R180 with a 150 Yamaha. When I picked the boat up from the dealer last year he told me that the motor doesn’t quite fit in the stern/transom area so they electronically stop the tilt mechanism before it damages the motor (or Boat).
If you plan to tow skiers or tubes, get the 150. The 115 will have to work much harder to keep up with a boatload of people, your gear, and the load your pulling. If you're just fishing the smaller motor will be fine.