Spirit is better while levelling than it is at level 80 - it's even quite OP. The reason is of course Spirit Tap. If your mana situation isn't satisfying, more spirit is probably the best way to solve it.
Int on the other hand is not as good while levelling as it is in raids on level 80. Especially if you don't even have Shadowfiend and Rapture.
Other than spirit the only stat that really effectively helps your soloing is spellpower (and haste to some extent but that is usually harder to get). Offensive crits aren't very exciting as disc.
I probably should, when I was still Holy(yes, I have a funny way of picking specs when leveling) I did get it in my build, and stacked at much spirit as possible.
I found it exceptionally useful. When I made my disc build, I did feel a bit annoyed I didn't get it, but then I figured getting Divine Fury for smiting would be interesting.
So I'll get spirit tap once I get Divine Fury..
I knew about the power of intellect when healing(my Penance actually costs like 40% of the mana cost it states due to rapture), but here I am actually talking about solo'ing.
Where I don't gain the benefit of Rapture.
So assuming that, spirit would be better, correct?
And even without spirit tap?(which I will be getting later on)