Take the 25 for an engine. The Sig line, in general likes a more powerful engine. BTW, your plane is only slightly smaller than the Kadet Jr(48 Span by 36 long), which has the same engine requirement, to which I was told is GROSSLY UNDERPOWERED with a 15. This was several years back, when I bought the kit, but it probably still applies. I'd look at an OS 25LA for the plane and go from there