How much sturdier are through mix pavers vs face mix?
A quick search reveals that face-mix pavers have a concentrated wear/color/texture layer on top. I don't see any indication that this substantially affects strength. If you install a proper base, strength probably isn't a concern anyway.

Prior to Face-mix technology, pavers were a blend of aggregate, pigment color,  and concrete pressed together.  When the thru-mix wear with age, the coloring fades and the coarse aggregates in the pavers (which are used to increase the strength of the paver) starts to show through the top of paver, which can ruin the look of the paver.
Face-mix technology adds concentrated color and fine wear-resistant aggregates on the top of the paver, in addition to the coarse aggregates at the base of the paver.  So not only is the paver structurally strong, but the color and finish of the paver lasts for generations.  The Face-mix process eliminates the risk of the coarse (and ugly) aggregate from appearing through the top of the paver.
https://unilock.com/paver-technology/thru-mix-vs-face-mix/

Of course, through-mix sellers have their own arguments:

If the concrete paver is brand new someone may not notice the difference, over time however it becomes obvious. If a face mix paver should become chipped the gray core will be exposed to the surface. The core of a through mix paver is the same color as the surface so this is not an issue. Since the color for a face mix paver is not mixed throughout the entire paver it tends to fade at a faster rate than a through mix paver. We manufacture all of our pavers using a through mix allowing for more intense longer lasting color.
https://castleliteblock.wordpress.com/2015/09/16/what-is-through-mix-and-why-we-use-it-exclusively-for-our-pavers/

It's really a matter of your priorities. If you don't expect rough treatment, face-mix might give better results over time. If you expect chips and scratches from hard objects, through-mix is probably the best choice.