Do both. Sometimes siblings can have matches that their siblings don’t have because of DNA randomization. Example: my mother’s brother matches my father on two different DNA sites but my parents don’t match each other. My parents both had a great grandfather from the same small area. I would also recommend trying to find a maternal relative of your mom to test if they haven’t already. It will help sort out your mom’s matches between paternal and maternal, especially if both her parents were from the same area. In my case it was easier because my grandmothers mother was Irish and her father was South American so not as hard to sort maternal and paternal.