Lesson?
I know the difference, and made the decision to go with the lesser ply for various reasons.
Marine ply is lovely stuff, but it is twice the price, which wouldn't be a problem on an 8 x 4 standard build, or even on an 8 x 5 if we could get 5 x 5 sheets.
I'm building a 9 x 5, and its almost impossible to get any other size sheet than 1200 x 2400 (3 feet 11 and 1/4 inches x 7 feet 10 and 1/2 inches) here. You get the odd imported sheet that is a genuine 8 x 4.
On this non standard build size, there is a lot of wastage. A lot. So I made the decision to go with structural ply. It has the one essential thing, the absence of which seems to let a lot of people down - waterproof glue.
The only real difficulty I've had is one side warping at the rear, which I partly blame on the way I joined the extra piece on. I reckon I should have equilibrated the two pieces to make sure they were both the same moisture before I joined them.
That same tendency to bend easy paid off on the front wall. Edge had dramas bending 6mm marine ply to a 600mm radius, the 7mm structural ply fell into place.
But yes, lots of filling to do before I paint the plywood.
But the filler (sanding dust and PVA) is cheap, and I'd have to fill all the screw/nail holes anyway, and marine ply over here is made from pine, just like the structural ply.
Unless you get the imported hardwood one, which is bloody heavy.
So bugger it, a bit of filling here and there, maybe take a bit of extra care to get everything square.
To be honest, the one consideration that can't be ignored is the price.
Like I said, its double. I've used 4 sheets of 17mm, three sheets of 15mm, and 7 sheets of 7mm plywood so far.
On a first build, with the potential for rookie mistakes, the price differential was a deal breaker.
I am not particularly wealthy, and I would have given it a miss if I could only have marine ply.
If I were to give advice to any Aussie builder it would be this:
Make the floor from structural ply or formply - it's getting a frame under it and won't go anywhere.
Handpick the best ply you can find for everything else as long as it has an A bond.
If your walls are going to be single sheets of ply rather than frame clad inside and out with thin sheets, stick with the 8 foot length and use marine ply for that.