Which IV Sigurd?

I say +spd. It will deny more doubles which will make up for the lost bulk imo.

I use a +Res -Def one (my other Sigurd was even worse) and he's doing just fine. Your +Spd will get to very respectable figures with Cavalry buffs and like said, will deny doubles for many units... which kind of makes Crusader's Ward useless.

While the other option will get doubled, the (ranged) damage drops drastically from the second hit and it could charge up a Special for him to retaliate with maximum force on his turn.

So it comes down to how you plan to use him and what sort of skills you will have, either IV set is workable.

I think the Sigurd with +Atk, -Spd is the better IV since the -Spd wouldn't really effect your Sigurd's fighting style, which is to tank from fast units' doubles. The huge Atk boon for Sigurd may give him a chance with Heavy Blade and can nuke others a faster charge.

This is just a suggestion and it's your choice for what's right. This is just what I think and Sigurd is such a great unit to get. :)