You will love the Suzuki DF9.9
Some things to know. The engine is EXTREMELY efficient and the lightest in it's class. It is quiet at neutral and 2/3ds power levels and up. Some people object to a mild gear case whine at low and trolling speeds it doesn't bother me but then again I didn't notice it until it was written about and that I'm usually at WOT except at warm up and 5 mph zones. If your boat is rated for it, go for the bigger engine. The 9.9, 15 and 20 HP engines are exactly except for the ECU (engine control unit) and the decal on the engine cover. All the other internals, gear ratio and prop are the same. The ECU controls the power level. So in other words the 9.9 isn't working very hard. The stock prop will over rev on the 20 hp unless you are using it as a kicker but it's good for break-in. Install an in-line water separator on the transom like the Sierra they sell at Walmart, the one on the motor needs all the help it can get. After the break-in interval is over, the power output rises. If you are running the engine deep like most RIB's (cavitation plate well below the boats bottom) you may see twin rooster tails. Some units have a 3/16-1/4" wide blunt edge on the front of the gear case. If you round off the edges, the tails go away. Use both the engine's mounting clamp screw AND the bolts provided. Yes I know it's a pain, but if you cross a wake and counter the forces to control the boat you can torque the motor right off the transom using the clamps alone! (Not just this brand of outboard) This is even more important if you mounted the motor high on the transom. You may have some tiller torque at high speed. Don't use the heading hold lever to control this because it vibrates the tiller handle. Use the tiller friction bolt but don't over do it. If you need more control you can make a wedge of "JB weld steel"on the right side of the skeg. Just remove a 1- 1/4 wide strip of paint from the gear case to the bottom of the skeg and build a wedge of JB weld to about 1/4" thick at the trailing edge. When done the trailing edge should be blunt/flat when veiwed from behind and even with the motor's skeg. Don't round the trailing edges. Sand off or build up how much trim you need and paint. The trim could cost you about 1/2 mph but what you gain in safety and fatigue is worth it. I didn't need them, but I highly recommend "Smart tabs" if you have slow plane or porpoising. Several guys have ditched the "Dolifns" and installed the tabs and are thrilled. I know it's a lot info, but when you have things fine tuned it's a lot nicer.