Well, personally I feel that Balor must have been much stronger. Why? Well lets consider some things.
1) Soulblighter (SB) only had one fallen lord (Shiver) at his disposal, and was only able to keep her because he raised her from the dead.
2) SB couldn't even keep the Deceiver (Big D) on his side, he went to the Light as soon as they rescued him.
3) Balor was able to bind even SB himself to his aid. Apparanly SB was origionally good, and Balor managed to corrupt him to evil.
4) Balor was able to conscript 3 who were formerly good. SB had none.
5) Big D stayed with Balor, and avoided SB, and we all somewhat agree that he tends to go with whichever side seems to be more powerful at the time.
6) The Watcher was on Balor's side (nuff said). He was able to defeat the MOST POWERFUL sorcerer in history (Mazzarin), and he was subdued by Balor.
7) Well, lets face it, Balor was the leveller (supposedly)
8) Alric had to use a stone which made him invincible to beat Balor.
I don't see how you can say that SB was the more powerful of the two. I think he was just trying to bring on the age of Darkness. They both must have been really weak if they couldn't do that, because it was their turn to win.