When it comes to refugium selection, it doesn't really make too much difference. You can even easily make one as a DIY project. Here is what to look for.
You want a certain amount of size. Ideally about 1/3 the main tank's volume, but it's not a huge deal if it's a bit smaller or larger. You want to avoid all these small hang on the tank units, since they are really too small for most cases.
Consider what you want out of the refugium. Some people want it to produce pods for the main tank. Others want it for nitrate and phosphate reduction. Still others want it so they can keep something that they can't keep in the main tank for some reason. All of this is more a function of how you set up the refugium, and not the refugium itself. However, this might effect what lighting and how much flow you run through it.
To prevent it from becoming a dirt trap, you usually want to feed the refugium from the cleaned water from the main return pump.
As you can see there are several considerations here, and you want to balance them so the end result does the most for your tank.