Left and Right Ascenders
Ascenders are sold individually, not in pairs. So make sure you are buying one right ascender and one left ascender.
Ease of sliding up the rope
Of all the features to look for an ascender, this is the most important. It is how easily the ascender slides up the rope that will determine how tired your arms get over time. If the device slides up effortlessly, your arms will thank you.
Ease of taking on and off the rope
This is the second most important feature in an ascender. Remember that no ascender is perfectly intuitive to use at first; it takes practice to be able to quickly get them on and off the rope. So if you are testing ascenders head to head in the store, make sure you use them a LOT. For example, at first Chris Mac could not get the nForce ascender off the rope quickly and his friend Tim could not get the Petzl off the rope quickly. Then they both practiced a lot. In the end, after lots of practice, they found them both about the same to get off the rope.
Weight is not a big deal when you are using ascenders. But you will notice the difference in weight when they are clipped to the side of your harness. Since big wall racks already way so much, it is nice to have a light ascender on your harness when leading. Also, if you are alpine climbing, the lighter the ascender the better.