Which is better overall?

I personally run +atk-Spd enemy phase mixed tank, but if you're running a mixed phase set then 35 spd is a great benchmark
+atk is better for player and for enemy phase
+Spd is better for mixed phase

I also use a +Spd/-Res and it works great. +Atk is good too though. He is a great cavalry to use, I still have him paired with The Reinhardt and they work excellent off each other.

If I had Sigurd I'd want him to focus on tanking, and getting hit less = tanks better, so I would go +spd.

Also, with hone cavalry, he would actually double any unit that isn't fast.