Divine Tyrfiring in Seliph is good?

You get about the same effect if you slap the Deflect Magic seal in him. Sigurd is generally considered a bit better since he has more movement and cavalry buffs, but as a mage tank they do about the same. Seliph is more easily mergeable, though, which also means you can get a better IV spread easier.

As you've said yourself, Sigurd has a much better spd and a better kit, not to mention generally better mobility and access to cavalry buffs. So... Yeah, Sigurd is definitely the better unit.

Also... Remeber that Divine Tyrfiring only reduces the damage from the first magical attack by half, and given how low Seliph's speed is he will most likely suffer follow up attacks. And since he doesn't have Crusader's Ward...

Having said that, if you really want to use Seliph, you might as well give him Divine Tyrfiring and DC. Just beware that, unless he is able to one-shot whatever mage attacked him, he will take to full damage from the follow-up attack.

Edit: Oh, yeah, I forgot about the Deflect Magic seal. I suppose you could it to him instead of DC.

If you give him steady breath, quick riposte, deflect magic 3 seal, he can become quite a monster. The ability to get SB is a nice advantage over Sigurd, though it doesn't make him overall better. Still, you can tank magic hits and charge your specials pretty nicely and if you invest in DEF, you can also tank physical hits too.

I think Seliph is better than Sigurd as a magic tank.
Here is my build:
Divine Tryfing
Aether
A: Warding Breath
B: Gaurd 3
S: Deflect Magic 3
In this build you want them to hit you twice, so your low speed is a plus. Being hit twice plus infantry pulse from an ally means he has immediate access to Aether, which restores the small amount of health lost, and helps him to take out tank mages(plus arena points).