I have a Saito 150 that has always run with APC 17X6 @ 9400RPM rich,peaked at 9700. (using Magnum 20/20)If the weight of the 150 is only an ounce or two more than the 120, go with the 150. It will not be too much weight for the UCD. I had a Moki 135 in one and currently have OS 108 in the second one.(I do add tri-stock to the firewalls). The Sa 120 is probably lighter than both of those and should be good power however the props required should be different than the 2 Strokes. I didnt get much thrust using 4-pitch props with 4 strokers,even if using longer blades. I like the 5-pitch wood props or 6-pitch APC best for 4-strokes.The 5-pitch Zinger wood prop actually gave me less RPM than the APC 6-pitch,same length?(I cut the tips on the wood props to match the APC curve)