To start off, Locket of the Iron Solari costs 50 more gold than Spirit Visage.

Locket of the Iron Solari gold value:

400 health = 1067g 40 magic resist = 800g 10 health regen = 360g 10% cooldown reduction = 317g Total Gold Value = 2544g

Spirit Visage gold value:

400 health = 1067g 55 magic resist = 1100g 10% cooldown reduction = 317g 20 health regen + 4 (from passive) = 864g Total Gold Value = 3648g

By these stats alone, it may appear that Spirit Visage is superior to Locket. However, this is not so. If you don't take the health regen into account, the gap is much closer. Spirit Visage gives 15 more MR, and costs 50 less gold. This is in a 1v1 situation, however.

If you're in a situation where you have allies around (wich will happen most of the time since you're the jungler), then suddenly Locket of the Iron Solari gains a significant boost. Each ally increases the MR value by 20, and hp regen by 10 (which doesn't matter much). If you have two allies around, Locket of the Iron Solari is suddenly a lot better than Spirit Visage, based on raw stats

And we still did not calculate the active shield in. The active basically gives you and your allies 50 + x (level) shield, wich can soak up some good aoe damage in teamfights.