It is easy to buy tickets at the airport - there are plenty of ticket machines and they take credit cards. There are rarely significant queues.
However, you can buy the tickets in advance for the same price, and it may save you a little time and stress when you arrive. Buy from nmbs.be and print the tickets, or download the NMBS app to your smartphone. The app supports electronic ticketing. You have to scan the barcode on the tickets to get through the ticket gates at the airport station - this works fine even from the smartphone app in my experience.
Personally I tend to buy a ticket in advance if I remember, as it saves a small amount of time and might be the difference between catching one train or having to wait for the next. It's really not a big deal either way, though.