For D.Tail not breaking Subs, consider this :
Giratina-O is asleep against Poke x (let's say Ho-oh) behind a Sub. I use sleep talk, it selects D.Tail and I break the Sub. This is something Giratina-A cant do due to lower Attack.
I know I can Roar out Sub Pokes anyway.
Also, I'm just assuming this. If you say, Giratina-O can't break Ho-oh Subs, ignore what I just said :p
I'm not using a spinner at all on my team. Also, I usually manage to prevent the opponent from getting up anything except SR. That being said, I think Giratina-A is the better option.