The formula for sieging ability, which applies to both the defenders and the attackers, is:
((siegers strength)²/100)+(bonuses(1 for flying, 1 for city guards etc IIRC))
If defending, mindless units have their value divided by 10. If attacking, they have their standard value.
For non-mindless units, the difference is that some units have superior defense ability, while some have superior siege ability.
It was outlined quite some time ago in
this thread.