I've had much better experience with the Regadon as a monster hunter/chariot smasher than the Ancient.
I take a Bastiladon in baaaasically all of my lists. Depending on the board & the opponent, it hangs out with the Regadon (making it I3).
Most monsters are I2 or less. Very few are greater than I3.
The ones that are I4 or higher seem like they face an uphill battle against a Stegadon to begin with (going solely off of their stat lines, mind you).
That, or they're far superior to both Regadons & Ancients.
Without a Basti around, you can expect the I2 Regadon to, at the very least, fight its opponent simultaneously - that's always better than fighting last, with special consideration given to late game combat (when most monsters may only have 1 or 2 wounds left).
Chariots are typically so fragile that the Strength difference doesn't matter.
Neither shooting option impresses me, but I'd rather have the range of the bow (solely as a playstyle thing). That said, the Bow does become pretty threatening at 18".
Even more threatening if GW lets the Chief fire the bow via Errata (only requires 1 crew member) - I'd consider taking one on top, maybe with Warrior Bane & a Potion of Strength. *shrug*
Anyway, my Regadon has walked away from 'Mo v. Mo' combat unscathed more often than not, getting all of its attacks in prior to its target with this setup.
The extra initiative point + monster hunter role may even justify 'Unstoppable Stampede,' though I haven't fiddled with that strategy yet.
I mean, a 10 point upgrade nets D6+1 Impact Hits followed by 5 attacks.
It seems pretty nice (especially if you can get a Wyssan's in there) but its standard attacks don't benefit from Multiple Wounds, unfortunately.
Still - the Ancient can be expected to kill its targets with Impact Hits more frequently than the Regadon, but any ensuing monster combat will also see it taking a few wounds.
So yeah - at its price, the Ancient doesn't seem very cost-effective in that role due to its lower Initiative. Especially true for EotG.
But it does hit harder, sooo...
YMMV