I’d never heard of these before. Judging by this link…
I don’t think I’d pay that much extra for the 1.65mm thickness vs 1.5mm.
As for the gantry tubes, I’d argue that thinner is the wrong direction to go. The outer frame tubes can at least be supported if you notice sag. The gantry moves, so the stiffness is basically from the tube. I think the notion is to remove weight, but these things really won’t move fast enough that it’s an issue. If it is, usually it’s a question of acceleration, not top speed, and that’s easy to tune.
For the Z axis, I’d say it matters even less, unless you plan on sticking the router way out the bottom, in which case we should probably be having a different discussion, first 
Good luck with the build!