I would not recommend steel, if you can afford it go with Poly. They will stop multiple hits, half the weight of steel. The reason why the military is using Ceramics and Poly plates is because the round literally explodes when it hits steel, you can get shrapnel in your face. I know allot fo steel fans will say they have a anti-spalling layer but poly is still the best option (if you can afford) and the impact is felt much more with steel. Poly is what everyone I know uses on the AZ boarder. The round is stopped in the center of the plate and not at the surface so no shrapnel, and the round is slowed not sopped dead so while it still feels like getting hit by a truck, it still less impact than steel.
Icon Tactical Industries
My Steel plated rig weighs 16lbs, my poly one wights 7.5lbs
Oh an Kevlar degrades over time and has an expiration date.
UPDATE: After going out and having a smoke I started thinking about what is the best option for SHTF scenario. While Poly is the best light weight option its meant for active use. When used in military and LE if they got hit in the plate, they could just replace it. In a SHTF scenario you probably won't be able to go online and order a new plate, so for SHTF scenario steel with an anti spalling layer is probably the best option or, a set of each if you can afford it.