I fully understand your will to avoid getting watch which are to close (as having a smae ref in 2 different material). I would of course act the same way, even if it is more to me, in the end, the fact to know
which one is the one I would like to wear most.
This being said, if it is a watch very similar to what I already have, then I would take it anyway.I have a 5712A too and if I was going for another Nautilus I would then go for a white dial (5726 or 5980). But this is very personal and there are so many choices in the Nautilus family that it is difficult to choose.
As I said in the past I love the 5960A (with a little change in strap though).
Now, to come back to your main question about the composite strap comfort, I haven't had the opportunity to try one during hot days (just in the AD).
I'm curious to see what you'll choose in the end

Cheers, Mark