I see this question come up in social media groups and sharing this video on some opinions after having a chance to own both.

KX040
Can haul on 7 ton trailer
Not need a CDL
Tighter places
Seems a little quicker
Not much it can't do the 57 does, takes a little longer
Quite a bit cheaper

KX057
Little more hydraulic
Little more digging power
Going to need a CDL to be legal
May be better for a mulcher