For most motor makers, usually a 9.9 is a full-sized, lower power version of a 15 to meet restrictions. In fact, a "9.9" cover can be bought for a Honda 15 outboard to fool the water cops. They are exactly the same sizes.
There are lakes where you can use a 9.9 and not a 15. The 15 probably won't get you much in terms of speed on a 22 either.
Tohatsu 9.9 and 15 both come with EFI. The old 9.9 carb model appears to be relabeled as a "9.8" HP.
My guess is that the dealer has an older carbed 9.9. I'd take Bob's advice and get EFI, 9.9 or 15.
I would go with the 9.9, but, then, I've already removed my Honda 9.9, tilt and steer, electric start, because of the weight on the transom. Very nice and I never used it.
I like an electric trolling motor better.