What you want, depends on how concealable you want it to be. There is no free lunch and armor that is truly concealable - as in can't tell you're wearing it in a suit and tie - won't have quite as much coverage up around the shoulders as the "concealable" armor using worn by uniformed officers.
If you're working as an armed security guard or uniformed police officer where body armor is more or less expected, it makes sense to lean toward more coverage and less conceal-ability. The bad buys are going to assume you have it anyway and the extra bulk that is common in soft armor worn for uniformed duty won't be a big deal.
If on the other hand, you plan to wear it in a plain clothes detective, executive protection, or other capacity where you do not want to look like you're wearing body armor, then you'll want to lean toward less coverage and more conceal-ability. If this is what you are looking for, then fit starts to matter, and you'll want to deal with a company that produces a wide range of sizes so that you get a good fit around the arms and shoulders. The goals is decent coverage without extra bulk, which means just enough coverage on the sides for the soft plates to overlap just slightly to avoid excess bulk. This differs from duty armor where the overlap will be about 2 inches. You also want to be sure the armor does not ride up into your throat when you sit, so that usually means 2-3 fingers worth of gap above your belt line. You also don't want it bunching up on your shoulders, so that means a little lower coverage than you'd have with soft armor intended for uniformed duty.
I needed mine to be fully concealable and bought my last set of concealable body armor from Defensive Products International and had it fitted by them. It wasn't cheap but at $529 it didn't break the bank either.
I opted for Honeywell Spectra Shield, rather than kevlar as it's sealed and moisture isn't an issue. It's also about 25% thinner so you can get NIJ Level IIIa protection for the same weight, thickness and conceal-ability as a Level II kevlar vest. The downside is that it is sealed and doesn't breathe, so a wicking undergarment is needed for comfort in hot weather.
You'll find some controversy around fibers other than kevlar. There was for a short time armor made from a product called Zylon, but it was found to degrade very quickly under conditions of heat and moisture and it was pulled off the market. There were also some Dyneema vests that were recalled shortly after that when the NIJ started including heat and moisture over a period of time in it's test protocols before re-testing to ensure the vest wasn't losing it's ballistic capability. Spectra got caught up in this controversy as Spectra was also a polyethylene derived fiber.
In essence the concern was with how polyethylene fiber vests react to heat. If you plan on leaving your vest in the trunk of your car where it will cook in the summer heat, then go with Kevlar just to be safe as temperatures higher than 180 degrees have caused delimitation in some older Dyneema vests.
I have fewer concerns about Spectra Shield and Spectra Gold vests, and I have no concerns about any vest certified under NIJ Standard-0101.06 (released July 2008).
In short, anything tested under NIJ Standard-0101.06 has been tested for water immersion, extended exposure to conditions similar to heat and moisture when wearing it and day long exposure to a temp of 194 degrees before being re-tested to determine in any degradation in performance has occurred. You'll find most companies have a 5 year warranty, but it's to limit liability. If a vest has passed NIJ Standard-0101.06, it's ability to stop a bullet isn't going to change with age, unless you badly abuse it.
I wear my vest, rather than leaving it in the trunk, and if it ever reaches temps exceeding 194 degrees, I've got far bigger problems as it means I'm also at or above 194 degrees. I treat it well, store it flat at night and consequently, while I've used mine for 7 years, it's still going strong - no de-lamination, no curling, no other issues.