Paralyze vs SoulSlay: MR mechanics?

[Chazar]

I am basically interested in success chances. So I figure that MR or MReasy is basically the same across all spells (subject to penetration and excess caster levels).

However, this is very interesting, which makes SoulSlay much less appealing:

Originally Posted by NooBliss

I have a feeling that Soulslay doesnt change your status to 'dead', it deals 999 damage and thus is countered by luck half the time. Paralyze isn't, so paralyze works 2x times more often.

Anything else along those lines one should be aware of?