[GTX JOHN]

Either one is fine. You are on the right track with triples and .040.
Make sure floor is 3/4 Plywood ( I prefer tongue and groove). Floor
needs to be on 16" centers = walls can be 24" but should be 2/2 Box
not angle.

I advise either 5200 or 6000 axles NOT 3500 even with triples
for durability.

Ask who their supply is on Axles/Springs related hardware.
My son uses ONLY Dexter Axles and components. If you inspect
closely majority of axles other than Dexter are sourced in China and
are 1/2 the price but a BIG gamble and potential Headache!

We speak from experience = WE have been building trailers for 30 + years!