In that case that will be determined mostly by whether you're traveling from Tarifa or Algeciras.
Ferries run every 2 hours from Tarifa, and every 30-60 minutes from Algeciras. From Tarifa it's a faster ride, as it takes you right to Tanger, while ferries from Algeciras go into Tanger-Med.
From Tarifa it's FRS, while from Algeciars they have a few competitors such as Balearia and Transmediterranea. If you're traveling on foot from Algeciras, there's no reason to pre-book tickets, just show up and take the next available ferry. Prices and travel time vary, but not by much (+20 mins for a slow ferry and IIRC there was a price difference of ~10€ between the different companies).
I think I've taken all 3 at one time or another, and they're not very different. You can pre-book tickets on their site, and that may save you some time, and maybe a bit of money, but you won't have the flexibility of a walk-up fare.
Hope this helps