Which one?

+Spd is probably better.

IMO I would go for +atk for the balanced 38/38 you would get, but the speed buff would help her double arya much easier, along with a few other fast ones.

Either is fine, you usually just use a sacred seal to increase the stat you don't have a boon in, such as using a spd seal if you use the attack boon or using an atk seal if you use the spd boon. So either way, you'd likely have the same outcome either way.

Although, go with +atk if you want to use heavy blade or something.

+Atk is better. Most of her targets have lower speed (Dorcas, Hector, Grima) and as mentioned by GoodHentai you could use the seal to give her more speed if needed.

Unless you run TA or heavy blade, I don't see how an atk boon is better on mages with great speed. I'd go speed boon with no hesitation.