whats the difference between chainfists and a power fist?

[Halifax789]

in terms of game effect?

[jackx]

Chainfists have a 100% chance to destroy doors with melee attacks, which comes in useful when dealing with jammed doors.
Destroying regular doors (instead of opening them for the same cost of 1 AP) also has the advantage that the Genestealers can't close that door on you later, but the GS AI in Ascension doesn't close doors anyways, so there's no benefit to this in Ascension.

Combat effectiveness is identical, both provide the same to-hit bonus, the chainfist does have its own set of animations and sounds, though.