People like VPS for FSC, because Vanaduke and trojans do a lot of normal. Elsewhere, VPS is usually not as good as specialized shields that protect from normal and one other damage type.

Sworders like BTS for a piercing shield. Gunners like Swiftstrike Buckler for a shield. So these people rarely use IPS. I bet that very few bombers use IPS either. So very few people like IPS.

It seems that you're developing your shield by (A) selecting a 2- or 3-star shield, and then (B) trying to figure out how to upgrade it. Instead, you should (A) pick which 5-star shield you want, and then (B) get whatever 2-star shield starts its alchemy path. That is, plan based on 5-star stuff, not 2-star stuff. This helps you avoid dead-ends, where none of your choices are ideal.

If I were you, I would do this: Don't get either upgrade. Just switch to a more useful shield line. You need to realize that most of the cost in making a 5-star item comes in the final upgrade from 4-star to 5-star. Therefore, even if you have the 4-star version, it's not too late to change alchemy paths. Don't throw good money after bad.

But I have a feeling that that answer won't please you. It seems that you really want to upgrade that shield. Then my advice is: Get VPS, because it's good in FSC, probably good in a couple of other special areas, and it looks cool.