I don't like special characters normally just cos there uber versions of the actual lord choices.
My friend is running around with a vlad isebela konrad and manfred vampire army. It is one of the gayest things i've seen in a long time (apart from all daemons hero lvl special characters and fateweaver)
There fine in big games cos it adds even more theme to the the battle your fighting. Say if u were playing like a 5000 point game archaon is far more likely yo be there
To answer your question i think sigvald is better as he makes his unit immuneto terrain and makes them stubbon ld10 which is very nice. I actully have a Valkia conversion for a campagin i was in there were lots of forces and everyone was using special characters so it gave each god themed list a bit more feeling. Anyway valkia isn't as hard as she first seems mainly cos she can't break a unit in one turn on her larries. Tbh neither can sigvald but thats not his job.