Size of the chair: The recliner should be such that it fits the person who is sitting inside it and hence you have to check the height and weight of your parents to know the exact chair that he might need.
Options for reclining: Apart from the system of lifting, the degree to which your chair reclines is also at your discretion. If you visit stores, you will find that majority of the lift recliners are sold as 2-position, 3-position or infinite position chairs. If they are more into taking a good nap, they would need a 3-position chair which reclines horizontally.
Features and style: You will also require choosing the kind of fabric, the back style, and the color which you want your chair to be. Do you want any massaging elements in your chair or a hugging chair? If yes, you may add such features as well.