I am a firm believer that if you have a slow cruiser, then it is a good idea to have a fast dink. Also, I like the idea of a light dink and motor. We have been using the Mercury Marine Dynamic 310 on our Island Pilots for years. It has a finished interior - weighs 100# - and goes fast with a 10HP (which weighs 84#) - so your total package is 200#. About $4,000 total. If you can't handle the weight, then there are two smaller Dynamics that take smaller motors.
In my opinion, the real advantage of a RIB is the ability to leave unattended w/o any fendering whether tied up to your boat, a friend's boat, or the dinghy dock. Also, they have great carrying ability with lots of reserve buoyancy and stability.
With two aboard, she planes off quickly - with one, she's a rocketship. With the 15 HP (more weight), my guess is she'll plane with four aboard? Never used the bigger motor, so can't really confirm.
The only reason I'd have a rigid dink would be if it could sail, too. The Walker Bay qualifies AND has an inflatable tube option that gives it the fendering ability of a true RIB. Never used one - just admired at boat shows.
"Sunshine" - Island Pilot DSe 12m