I suppose when it gets right down to it, that your intended purpose/project size will have a bearing on which engine might work out better for your purposes. To that end, been playing with the 10 for a while now, and have had it in several different planes. I can tell you a good target for the 10cc engine with decent sport performance in mind will be .45 size plane around 6lbs and 650 sq in wing area.
Not sure how much smaller you would have to go with the 8cc engine to get duplicate performance.
I can tell you that both engines use the same carb.