I have a 9.9 on a fiberglass rib with a flat floor.  It does come on step with me alone. Anymore weight I doubt it would plane.   I’m a big guy so take my experience for what it’s worth.  If I replace the current motor I’d go with 15 which is the max.  If I only wanted to slow put to the dock maybe I’d go sub 5.  

We have a 9,8 Nissan 4 stroke. We use to have a 9.9 2stroke yahama. The fuel consumption is about 1/2 of what the 2 stroke was. Power wise it's  on a 9 ft aluminum  AB rib. We can plane with 3 of us that are 160 each. Weight wise maybe  2 or 3 lbs more. 

Our 8 1/2 foot RIB with deep V weighs in at 120 pounds. With some effort, it planes with a Merc 9.9 HP carrying about 340 pounds of weight (two adults and a small dog). With one adult, planing is no effort. I'd be tempted to go with the 15 Hp Merc with your 10 foot RIB. Other than speed, another consideration is mounting the engine and removing at the begining and end of the season. For my age (63), the 84 pound weight of the 9.9 is about my limit to move it around without help. The 15HP Merc is another ~30 pounds of awkwardness.

The Merc 9.9 and 8 HP models are one in the same except for carboration, and weigh in at the same 84lbs...I would go with the 9.9.

We have a second engine- Tohatsu 3.5HP - it pushes us fast enough for most of what we do. Being a one cylinder engine, it does vibrate a lot (compared to our 2 cylinder Merc) and obviously, it doesn't come close to putting us at a plane, but it's ~40 pound weight makes it easier to handle.

One last tip. The new 4 stroke engines are prone to gasoline related issues. The 10% alcohol/90% gasoline formulas commonly create fouling problems in marine four stroke engines. A good way around the issue is to burn aviation gas if you can locate it. We live near a small airfield and they have no issue selling us small amounts of aviation fuel. In fact, they have lots of outboard motor customers. Aviation fuel has no alcohol.  It costs more, but since I've converted to it, all the fouling problems have disappeared and I no longer spend my time pulling apart the carborator to repair it.

I have two O/Bs for High Flight.

For Columbia River cruising and other protected waters, I have an electric start Merc 15 hp, because the Admiral doesn't/can't pull start an outboard. It planes our 10.5 ft RIB with both of us onboard. Alone onboard I have asked a nearby PWC to give me a speed, and he said 21 knots. Center of Gravity is critical to planing. Use a tiller extension and place the fuel tank and battery as far forwrd as you can to get the RIB to balance effriciently. This makes a super proetcted-water package. The electrical system allowed me to fabricate full LED night lighting so I can use it safely in the dark. The downside: it weights almost 140 lbs. I don't want an engine hoist (I have one for sale, btw.) because it adds even more clutter to the davit-equipped stern. I carry the RIB/OB combination on the davits in protected water, but I would never go out in the ocean with that weight hanging there.

For my Alaska trip last year I bought a 3.5 four-cycle Merc. Only 35 lbs, I am able to lift it to the stern rail pad while I stand on the swim platform. Not a chance this will plane the RIB. It is a utility powerplane only, but does support using the dink for trips ashore. etc. 
 

Enrique;
 We're using a 3.5 HP  two stroke Tohatsu to push a 8.5 ft. Achilles aluminum bottom dink around the anchorage.It does a fair job of pushing it about and will actually get the boat on a plane with two in the boat.. I think the things to consider are just how much weight (people)  will normally be in your  inflatable and how much engine weight you are willing to put up with. We have a Garhauer crane and a walkthrough transom. We did have a 8 HP Honda for another dink but its 90 lbs was more than we were willing to put up with. Way too much weight for me alone to handle. So went to a smaller dink and smaller engine. We'll give that a try for a few years.
Seems to me a 9.9 should be just fine for your application.

I agree with Bill about the weight... slightly more HP and the weight goes up. Its my understanding that Tohatsu makes most of the small engines for the other manufacturers so if you go with a Merc or something like that you might find the same size Tohatsu will be cheaper and you'll get the same engine. When I replaced mine outboard I ordered it on line and got a relatively large savings going with Tohatsu

The PO of our boat purchased a new Tohatsu 3.5 for his dinghy. He could never get it to run steadily and gave it to us after maybe an hour of operation. After a can of carb cleaner and change of fuel, it reluctantly ran but not with high confidence (which I need becuase of  river current we are moored in.) I tried aviation fuel (100 octane) as noted in my above thread and it ran just fine through the entire summer. I also didn't have to run the carborator fuel bowl dry with each use. Alcholol absorbs water and leaves residue in the carb jets. The aviation fuel has no alcohol. We purchase it from a local small airfield. It is also available at some gas stations like Haffners, but they charge an arm an a leg. 

Enrique...

Perfectly happy with the Tohatsu. I have an 8 HP with forward & reverse and have never had a problem with it.

I would agree with the Tohatsu selection.  EFI on a 9.9 would be great on a 10' RIB.  Fairly heavy (106lbs) but great fuel economy. It's new tech and has a 3 year warrantee so how can you go wrong.  And the price seems good too!  Suzuki is the only other outboard I found that has a fuel injected 9.9.  Tohatsu makes the smaller Merc's but Mercury doesn't have that option yet. 

I have a 5 horse Honda and 6 hp Tohatsu. I like the 6HP better. Both will plane with trim tabs installed. Will not plane with weight over 425lbs. Hard bottom.
​See picture. Tabs were best thing I ever bought. about $100.

Tell us more about the tabs... were to purchase brand...

www.maximarine.com/products/lifters/

Maximarine lifters. Made a night day difference in handling and getting on the plane. Couldn't plane with one person and 6HP. Better mileage too. Takes about 1 minute to put on or take off. Seemed kind of cheap, but I have been using them for a couple years with no problems.
Looks like they are $114 dollars now. 
I do have to remove them to put wheels down. 

These things enabled me to plane without dealing with a heavier engine. 

Thanks Chuck