Good question.
I enjoy both songs. Mother Of Mercy's chorus is not sung good at all by Bruce (one of his ''bad'' studio takes, iirc he wanted to change the chorus, rightfully so), while Lightning Strikes Twice's chorus could have been with backing vocals. Both ideas work for the songs though.
Which chorus I like more? Dunno, maybe Lightning.
As songs, both are shorter with little time to get the chorus going, both sound like they could have been on the previous albums, and while I like them almost equally - I prefer Lightning for the melodies/harmonies, tempo and instrumental section. And I guess it's more unique in the discography, to say. The ''rocker feel'' is done great. I think I liked Mercy better when the album was released, although the dark feel and heavy gallop is superb.
Btw, Lightning is cool live and I think Mercy would have been better live if they had played it. Bruce would have done a better job with the chorus in 2011.