Avoid a material called "Zylon". It's found in some higher-end Second Chance vests, and is the subject of a recall.
"Level 3A" is the highest level of protection you can find now in soft body armor. It will be heavier and bulkier than Level 2, and Level 2A (in decending order of effectiveness). HOWEVER: newer materials are lighter and thinner than those of the past at the same threat level, so buy the highest level of protection you can afford. Vests only weigh 5-10 lbs anyway.
Take the proper measurements to ensure a good fit.
Buy the model that says "extended" fit. That's where it wraps around your sides more.
Don't forget the "trauma plate". That's an insert carried in the front of the vest, covering the sternum. There are hard and soft plates (or Pacs) made of metal wrapped in ballistic fibers, or of many layers of fibers bound together. The metal plates are a little heavier, but offer a lot more protection.
Don't get ballsy because you have one on!