Choose an airline fare hold when you already know the flight you want and the airline offers a suitable hold. Choose a specialist temporary-reservation service when your main job is to arrange an itinerary document for a specific planning or application need. Neither option is automatically better. The right one depends on the route, how soon you need the document, and what you expect to do with the booking afterward.
You need a flight reservation for a visa file or an onward itinerary for a one-way trip. An airline website offers a fare hold; a specialist site offers a temporary reservation. Are they solving the same problem?