Have played around with the math on this a bit.
Against vehicles always use Cleaver
Use Gauntlet if wounding on a 4+ or worse
Use Gauntlet if AP headroom is 0 or -1
Otherwise use Cleaver
AP headroom is a term I use for the difference between armour and invulnerable save, e.g. 2+/3++ or 6+ have a headroom of -1, and a 4++ has a headroom of 0.
Most targets Grotesques wound on 3+ or better, and you get full value for the AP on the Cleaver, so usually the attacks should be made with Cleavers.