You need to research each plate by each manufacturer. Armorwear has had horrible plates and awesome plates, same for AR500. NIJ certification does not mean much, IMO unless you require federal grant money... that is where the artificial 5 year warranty comes from too and some of the cost.
Having owned both steel and ceramic/UHMWPE, I'd never buy steel. IMO you want a stand alone, triple curved plate with a ceramic face and bonded composite backer in level III+ (if you want lighter more expensive) or level IV. You want a plate that is sized to fit your body and your carrier.
One the ceramic front, you have cheaper Aluminum Oxide and lighter, super expensive Silicon or Boron carbide.
On the composite backer, you have Fiberglass/ E glass/(cheapest, heaviest, weakest) Kevlar/Aramid (mid on all) and Dyneema/UHMWPE (lightest, strongest, most expensive).
For steel it's generally all AR500, flat or curved, very few multicurved. With no build up, slight build up, or heavy build up of a bedliner type spall reducer. Or you have to use a spall insert to catch spall in front of plate. A steel plate with spall liner in front and padding in back suddenly is not so thin or cheap as first thought.
Remember anything but SAPI cut is non standardized and is playing a game of less coverage to have less weight and price. Don't be fooled into thinking shooter cut or swimmer cut or whatever marketing term is providing better utility... it's just less square inches of coverage to save weight, material, and money.
I'd recommend not considering non or single curved plates if comfort means anything to you at all.
Steel plates are good for one thing, IMO... making you appreciate the multicurve ceramics.