Here are two things to consider if you need to get replacement chair covers.
Whether or not the way your household uses the chairs has changed
The first thing you'll need to think about is whether or not the way you and your family use the chairs has changed since you bought the previous covers. For example, if your children routinely sat on these chairs whilst making finger paintings and when (messily) eating their snacks when they were younger, then the previous covers may have needed to have been wipeable and robust, but not particularly beautiful, simply because of how regularly they got stained and grimy. However, if your children are now a bit older and you feel confident that they won't, for example, wipe the ketchup they get on their hands onto their chairs whilst eating dinner, then you should get chair covers which, whilst they'll still protect the furniture they're put on, are more stylish and delicate than their predecessors.
Similarly, if you had a dog who used to take daily naps on your chairs, but they have since passed on, then you could probably pick prettier, lighter-coloured replacement covers, as you won't have to worry about them getting ruined by paw-print marks, drool or fur.
Whether or not you'd like to customise the chair cover
If you're interesting in sewing, are quite creative and generally like to add your own touch to the decor you buy for your home, you should also think about whether or not you'd like to customise the chair covers. If you want to do this, then it might be better to get ones that are more suited to the particular type of customisation work you're thinking about doing to them.
For example, if you'd like to add your own tufts and upholstered buttons to the covers you get, you might need to pick a set of covers which are made from a reasonably tough fabric (such as a good-quality velvet) so that the covers won't tear when you gather bits of them together and pull them taut to make the tufts.
Conversely, if you want to embroider the covers with an image, it might be better to get ones made from a soft material, like cotton, instead of canvas, which is quite thick, as it might be difficult to push a delicate embroidery needle through the latter and the customisation could, therefore, take much too long.
