I look at a number of factors when deciding between Special Charge Up and Tenacity as the head slot's main ability. Now we're going to assume whatever weapon set you're using has a special you'd like available as often as possible without sacrificing all your slots for it . Next we'll take a look at each abilities brand name sub chance to see what each one has to offer.
Tenacity - Special Saver, Quick Respawn, or Special Duration Up. Of these three, the last one is far and away your best bet. The problem here is, even with three subs it's still only a negligible increase of the time your special is active save for Echolocator which is already twice the length of all other specials and has an ability that hard counters it. To put it in perspective, three subs will only increase the duration of specials like the Bubbler and Kraken by a fraction of a second. Hardly a difference maker.
Special Charge Up - Ink Saver (Sub), Quick Respawn, Ink Recovery Up, Strength Up, or Swim Speed Up. Of these you've got a few more options to choose from at least. You'll likely already have swim speed subs on your shoes. Ink Recovery is the most useful as it's active all match regardless of your choice of weapon set. Again this part is really about taking into consideration what sub abilities the piece you choose will likely be saddled with and how you'll incorporate them into the rest of your build.
Next and this is important is whether or not your main weapon is good at actually spreading ink. See all Special Charge Up does is lower the point threshold requirement for gaining access to your special. In other words, if your weapon is average to bad at covering turf, even it won't make up the difference it lacks in that area. I'd only advise it for weapons that are the very least good at it if not great.
This is where Tenacity comes in. This here is your ace in the hole when you want your special more often despite your main weapon having lackluster ink coverage. Despite only being active in small few second bursts, these periods are much stronger than what special charge up offers, but they of course come at the high price of both inconsistency and well your team being at least one inkling down. It's simply not an ability you can rely on.
As a quick example. Using the Forge Splattershot Pro and want your inkzooka more often? This weapon not only has poor spread, but has high ink consumption as well. It just plain lacks the tools to make proper use of Special Charge Up so therefore Tenacity is the way to go. It simply depends on your main weapon.
