All things being equal it is better to buy from the airline, but often all things are not equal. If you need to change your ticket you will have to pay the airline and agency fee and wait
​​​​​​on hold for hours. If you won't need to change the well known OTAs are fine. Stay away from ones you have never heard of. You will be treated the same in the event of delays or cancelation that are out of your control.