15mm vs 18mm Birch ply?

Is there any gain to be had from building a speaker with 18mm birch plywood instead of 15mm? It'll be stiffer but it'll also weigh more.

I can't seem to find any real comparisons, most discussions revolve around MDF or Plywood, not so much about thickness.

I'm in the middle of the basic design work for a 2-way speaker. BR 15" woofer + 1" compression driver in a waveguide.

The price difference isn't all that great but I imagine the difference in weight will be noticable.
The question is if there's a audiable difference?

And while we're on the subject of thickness...
What's a reasonable and efficient baffle thickness? I'm thinking two layers of the same material as the rest of the box should work?
For something the size of a 15" woofer cabinet I wouldn't make it from less than 18mm. You will also want to add braces to it to ensure the freeboard areas of the panels are smaller therefore driving up the resonant frequency of each section to eliminate the chance of the woofer exciting those panels.

Doubling up the baffle is common.
It depends on the type of bracing. There are basically three types:

1. Strap type: Where you are gluing wood onto the surface. ie: a 2X2 glued to a panel.
2. Stick brace: Where you run a piece of 2X2 square or round or whatever, one side of the box to the other.
3. Shelf brace: Where you make pieces the same size as the inside of the speaker [like a shelf] and cut a bunch of fairly large holes in it to allow sound to pass through. The shelf brace is highly effective but they use more wood, they take more time and they use up more cabinet space so they have to be accounted for in your volume. The amount and type are be dependant on your cabinet. You haven't told us that yet.
I've been thinking shelf braces of some sort.
The cabinet that I'm running sims for is in the 90-100L range.
Something in the vicinity of 40x20x12" on the outside.
So, fairly wide but shallow. Woofer mounted just above half way up and the port close to he bottom. I might go for twin ports?
Nothing really new and revolutionary. :)
I'm just trying to avoid the worst no0b mistakes.
It's all about comprimises I guess?
First priority is the audio but there's always a point where the diminishing returns kick in.
I try to avoid overdoing things just because I can. I'd much rather go minimalistic as long as I can still get great results.

Let's say I can get 99.5% of of a 100% soundwise, I can't hear the difference, it's 2/3 the cost and 2/3 the weight. Well sure... I can live with such a compromise.

Otoh, if the difference is audiable I'll just get annoyed and suddenly it's just a waste of time and money since I'll have to redo the box with proper material.
In this case I guess the question becomes "can you tell the difference between 15 and 18mm?"

Cal appears to believe you can and until someone steps up and claims the opposite I'm inclinded to believe him. :)

Sure I'd like to go light weight but not at the expense of the sound quality. The driver mounting flange is 16mm so 2x18mm feels reasonable to me since I'll be recessing it.
After modelling the enclosure for a while I think my initial assumptions might have been a little light sizewize. I'll probably have to add a couple of inches to the width and/or depth.

Shelf bracing to begin with, possibly something akin to what Dave (planet10) is using in his builds. Maybe som extra dowels or an extra horizontal shelf if the need arises? That's my current line of thought.