With the railroad tracks in @jtbell 's photo (beautiful picture, by the way), I could only think of the following Simpsons episode while reading your post:
Comparing both designs for buckling:
$$\frac{\left(\frac{5.5^4 - 5^4}{12}\right)}{\left(\frac{4.5^4 - 4^4}{8}\right)} = 1.255$$
So your design should be stronger than theirs.
Comparing both designs as a cantilever beam with a load at the end (for any horizontal load at the top of the swing)...