In 2001 i bought a wetline SIB, but couldnt decide whether to get the 290 or the 310. the 290 was rated at 10hp, the 310 at 15hp, and at the time i didnt have an engine. Money was tight and there was £100 difference in the cost, so the chandlers let me take both home to decide.
Once inflated, there was actually only about 3 inches difference in the length, so I went for the cheaper option, and bought an old Mariner 15 which i beleive is a re-badged yamaha. It is exactly the same size and weight as the 9.9 which my friend has. I have just upgraded to a Honwave 3.8 with air v floor, and am still using the same engine so I'm glad i got the 15hp.
I will say however that I bought the 290 with an inflatable floor under which was the inflatable keel, and the first time i opened up the engine, the transom kicked in at the bottom, and the floor started to buckle up so i bought a ply floor for it, and put the inflatable floor on top of it. Never had any problems after that!
I have since realised that in all the years I had the 290, I dont think that I ever had it inflated to the correct pressure, as the high speed inflator I was sold at the time for about £70 only went up to about 0.18 Bar when it should have had 0.25 Bar. With the correct pressure it may have been OK with the inflatable floor, who knows?
If I was you I would go for the Yamaha 15, but be careful-remember you dont have to open it up all the way!
I hope that sharing my experience helps, All the best.