well i've never flown the 120 with a 120 FS. I do have a 4*40 with a 52 surpass. Flys very well. will hover for a second or two and i'm at 4500 feet MSL. thats with an 11x7 prop. Anyway the manual says that a good 120 will fly the plane very well. It's not a 3d plane so you reall don't need a 150. Wan't my advice. Fly it with the 120 on it since it's all set up to fly that way. If you don't like how it flys try some different props first then if you really want more preformance then go through the task of setting up the plane with the saito 150. Personally i hate swaping engines because it take some time plus you have to reset the throttle linkage and so on. Plus every time you redrill holes in a motor mount you weeken it. Then you have to re set the cg. let me know how it flys.It should be fine with the 120.
-Matt