Depends on the size of the knife and if it will have bolsters or not. Generally my go to thickness is 0.040". On larger knives 0.050" can look a little better. The scales provide a lot more stiffness than the liners so I would happily go thinner on a shadow pattern knife compared to one with bolsters.
I have only dabbled in slippies, and use what I have on hand. .050 Ti mostly, because I can color it and I also have some .040 410 stainless that looks good for what I do.