I agree "stick to horizontal". A few nights ago I watched my DVD of "Stagecoach" where in the last few minutes of the movie John Wayne as the Ringo Kid went belly down on a dark street when faced by three bad guys who stood vertical a few yards away. With his Winchester Ringo sent each one of them off to meet their maker. If I'm ever faced with such a situation, I'd want to present as small a target as possible and in the dark if possible just as Ringo did that dark night.
I would go hard plate level 3+, it can stop 5.56 green tips. There's allot of that stuff out there now. You have to decide if weight and/or multi hit is more important. Most ceramic produced now is multi hit too though. AR500 and AR650 can take a few more hits than most ceramic.
I have several level 3 carriers and some AR500 plates but they sit in my backup stock.
I run ceramic level lv large sapi plates made in Canada with a JPC rig and HSGI gel shoulder pads. It weighs 20.6lbs with 3 loaded HK53 mags. It's light weight and very easy to shoulder a firearm comfortably(the most important thing for me). My next setup will be a Crye SPC with Hesco 3615 plates.