I've built the Goldberg and GP, but not the Sig. I have flown all three.
The Sig is structurally weaker than the other two. The strongest and to
me the best flying one is definitely the Goldberg. (I won't argue the point,
it's just my opinion.) Not hard to modify it to make it more scale, I did this
with my last one, barn door ailerons, etc.
A 70 four stroke works exceptionally well on this size cub, smaller engines
can be used but the 70 allows full aerobatic use of the clipped wing.
Again, just my 2 cents.
Jerry