Well, it stacks indeed with the armor but in case you really come in a heavy situation, switching to real S&B mode is probably better, especially then having a shield with divine whatever potency on it and probably even higher shield bonus then +4 and for the protection from magic missiles etc you can use items like the mantel of the worldshaper or others. If I am not completely wrong there are even pots with the 'shield' spell. So the need for this single lvl 1 wiz spell was questionable.
The extra amount of SP you get from a lvl 1 wizard compared to what you get from an additional fvs level is I guess not the big of the difference if at all, as for SP most splash a Sorc instead. Also I am not sure how good are the divine Enhancements you may loose that you can't access due to lower levels in FvS like
- Favored Soul Energy of the Scion IV
- Favored Soul Improved Spell Penetration III
- Favored Soul Extra Action Boost II
The free Metamagic feat would be the only reason I could see, but well, as I said, I am not sure if the sacrifices you do by getting a Wiz. level (e.g. lower DC for all your spells) will be worth that extra feat.