There is a good reason level IIIA is the most popular protection for police use. Level 3A will stop damage caused by most guns and knives used on the street or with domestic violence calls, IIIA can handle a 9mm 357 Magnum 44 Mag and even a shotgun shooting bird or buckshot. I would not trust it to handle sabot slugs from a shotgun though. There is a lot of power there.
If you live in big game country like Idaho, Montana, Wyoming, or Alaska, I would wear a level IV bullet-proof vest with a trama shock plate. The reason is high powered rifles out number handguns in these states. You need all the help you can get if the crazy zombie is shooting at you with a 270, or 30-06 Springfield.
For many years Kevlar has proven to provide the best ballistic protection. Dyneema and Spectra fiber has come on the scene the last few years and is stronger, cheaper, and lighter than Kevlar. You may have already used Spectra fiber in braided fishing line like Power Pro.