I am not saying which one of the two is a better motor, but I think the Saito is more suitable for UCD3D. Most people find that this plane does 3D best with CG @ 5 1/2" or further aft. With the Saito 100, it's at 5 3/4" for my set-up w/ three 2-oz servos at the tail, 5-cell NiMH battery above trialing edge, and no wheel pants. Total weight is 7 lb. YS will give you better throttle response than Saito for the same prop. But we can "buy" throttle response by using lighter props, such as Zinger Pro, which is not too bad, only 400 peak rpm less than the APC on a 15x6. However, it's not saying the APC 15x6 @ 9500 is generating 4%more thrust than the Zinger Pro @ 9100. It may very well mean that Zinger Prop, being wider, provides the same thrust @ a lower RPM.

The advantage of YS is that you can put fuel tank at CG and have a consistent CG throughout the flight, w/o adding a pump to the engine. Afterall, if you have to add 2 oz of tail weight, the wing loading is still phenomenonally low. Now, putting a 31oz 1.20 4-stroke is another story, as it requires putting the battery pack at the tail, and make for hot landing due to a high-thrust idle, unless you use the 16x4W prop on the 1.20.

I was having the same dilemma about these 2 engines, until I bidded the Saito on a popular bidding site. That price difference made my choice easy. I'd say if you find a good deal on either engine, jump on it. Hint hint?