First off, all of the common brands of rescue vests are quite suitable for rescue work.  Every rescue vest design must be approved by the US Coast Guard before it can be brought to market.  One major difference between the major brands is pullover versus a zipper entry.  Contrary to false advertising, all current vests do a great job remaining on the rescuer in heavy whitewater if properly fitted and secured.  Fit is real important to check out by trying one on.  This is especially the case for women whom lack the typical male V torso.  For many, pullover versus zipper entry is a matter of taste.  From my experience, the pull over types can catch on rear entry dry suit zippers and destroy them.  Zipper entry vests are almost always easier to put on which is why they are far more common.

The next major difference is the thickness of the belt and how easy it is to release.  I recommend wider belts as this spreads the load over a larger area and are generally stronger.  Also inspect the tri-buckle, a wider gap in the openings makes it easier to release (less potential friction).

Many of us use a rescue tether.  These come in two flavors - fixed or shock cord.  Each has their adherents, choose whichever you prefer as both do the job quite nicely.  The shock cord variety is faster to secure after a rescue.  Speaking of tethers, a large opening for the non-locking carabineer is great for looping around a paddle shaft.

Pockets are the next concern.  Personally, I like a large pocket so I can safely carry a 50' throw bag in the vest.  Extra pockets can be used to safely store carabineers and other gear.

If you can't store your rescue knife in a convenient pocket, you will need a convenient lash tab on the vest within easy reach.

Padded shoulder straps are a really nice feature, especially if you need to carry a boat some distance.  I also like climber loops that can safely hold extra locking carabineers.

Astral has a nice Spectra loop on the bottom of their vest that enables its use in vertical extraction.

Another point to consider these days is road rash.  Imagine flipping in a shallow slide rapid.  Very low profile vests leave the whole upper half of your back exposed - ouch!  Personally, I like a bit more coverage if possible.

In all SWR classes, we do a gear check on both the helmet and the PFD.  The PFD should be checked for obvious signs of wear like tears and extensive fading.  If the PFD is a rescue vest, examine the quick release belt for proper threading.  Look for any dangling strings that might catch on something.  Carabineers near the collar bone are a common safety hazard that is often overlooked.  The tether attachment to the belt should also be inspected.  Ideally, it should be a continuous ring from the manufactuer.  Alternatively, a locking carabineer can be used (never a non-locking carabineer).  Finally, pull firming on the shoulder straps to ensure the vest is secured snugly.

When I check a helmet, my main concern is fit.  I like to test for rollback with two fingers lifting on the brow above the forehead.  If the helmet rocks back more than a half inch with light pressure, it is far too loose.  This is a very common cause for concussions.  Another useful test is the Leroy Jethro Gibbs (NCIS) head slap on the top of the helmet.  If that really smarts, chances are the foam isn't thick enough or has little or no give.  No travel in the foam along with a stiff shell transfer the shock load to the neck (or head as well).  Don't hit the head too hard or better yet, have the student smack the top of their helmet themselves.  Better to discover this way than when you flip and hit a rock at speed.

As stated before, the most important feature of a rescue vest is the quick release system.  A belt is sewn into the vest and passes through very sturdy sewn in belt loops.  The belt is typically threaded through a metal friction buckle that takes stress off of the plastic quick release buckle.  The belt finally passes through the plastic quick release buckle which has a large plastic ball attached making it easy to grab without seeing and pulled for fast escape.  A sturdy ring is placed in the middle of the belt behind the PFD.  You can attach a tether or rope to this ring.  In a release, the belt slips through the belt loops and the floating ring and just the ring falls off (attached to the rope or tether).  This way to don't have to cut yourself free of the rope (always carry a rescue knife anywhere that is easy to get to). Here is a short video from Zorn Outdoors describing the quick release system: Rescue Vest.

The above video covers the typical way to thread the buckle.  I would call this the fully loaded option.  Depending on the jacket, some may experience release challenges under lighter loads like towing a boat for instance.  An alternative is threading through just one opening of the metal friction bar.  This method provides far less friction than the double thread and still ensures a nice parallel entry to the plastic buckle.  Unfortunately, some instructors will recommend skipping the metal friction bar altogether when towing boats.  I strongly advise not doing this.  For one thing, the plastic buckle isn't designed for heavy loads and may break and jam.  Also, the friction bar helps to prevent a pull on an angle which can also cause a jam.