This is interesting info about the current generation of OBs...nothing like having an expert chip in. Your information about the 6HP being a single cylinder is surprising and disappointing...I have not been much of a fan of the switch to 4 cycle motors...more wieght/more cost/less power. It all makes me treasure even more the Tohatsu 8HP 2 cycle I have on my RIB.
All given, I'd say look hard for a used 2 cycle 8HP rather than pay the cost and live with the weight of a new 9.9.
I don't want to start a 2-stroke vs 4-stroke rampage, but as a Tohatsu dealer/mechanic, I can offer these thoughts:
The old 50:1 pre-mix 2-smoke engines are excellent for their intended applications. Ours seemed like it would run forever, and the new owner likes it a lot. They are definitely lighter (size for size), but they are noisy, smokey, and burn fuel like crazy. And they do go through plugs faster, too.
As I mentioned, I replaced our 1974 OMC 15 with a new Nissan 9.8 4-stroke, and couldn't be happier. Much quieter, uses half the fuel, no smoke, no oil to buy and mix, and no annual change of points. Since we have it in the lazarette, we use remote controls, so the electric choke is a big plus. The old 15 did not have that option, so I had to run an additional choke cable for the old motor, and needed both hands to start -- one for the key, and one for the choke. Of course, this is not apples to apples -- heck, it's not even apples to broccoli. They are completely different design generations, as well as different combustion technologies.
Thanks to the Solas high-thrust prop, we have better thrust with the 9.8 than the 15 ever had... and it BACKS UP now. That's something the old setup just did not do well. Because of the better thrust, we actually saved weight, by going down one motor family size. The new 8/9.8 family is lighter than the old 9.9/15 family.
Each setup is different. In our particular case, on this particular boat, the 4-stroke was the way to go. I like the simplicity and weight of the 2-smokes. I have one on the dinghy, and intend to keep it "forever". If our sailboat aux was out on a bracket, the 4-stroke would lose many of the advantages it has in the lazarette...