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.