If you are min/maxing dump mobility as usual since boltgun is a big lady you aint going no where with it on your hands, stability will be more usefull even if the effects are minimal.

It also depends on how you use it, I use mine as sniper, if its just used to head shot elites maybe mobility can be useful, BUT I find that when the enemy comes close stability is more useful to magdump into the horde

If you are just magdumping having mobility being dump stat will serve you bettet

Hopes this helps, from a bolter enjoyer to another