I can't speak for those champions specifically, but for support champions, I normally take Scorch on supports like Nami and Soraka for more harass in lane, and take Gathering Storm on Brand and Zyra so I scale better.
I think it mostly depends what you keystone is. Most people are playing Annie with Predator/Electrocute, so if you are specing into Sorcery as a secondary, I think Ultimate Hat and Celerity are better for more ults and helping you get in range to ult (plus more damage when you predator).
Assuming you are taking a keystone in the Sorcery tree, then I think it mostly depends on the matchup. Both scale well with AP, so Gathering Storm will be a good choice regardless. Of course, that's assuming the game gets to that point. As someone else pointed out, you basically have no rune for the first 10 minutes of the game. Most of my games though last 20-30 minutes, so I get a free Amp tome for just existing.
If you are against someone who you can poke constantly, then Scorch will help you get an early lead, but that's assuming you can do something with that lead so that you don't regret taking a rune that only does 60 damage every 20 seconds while your abilities do 200 base damage with AP scaling on a much shorter cooldown than that rune.
Another aside is that Scorch only works on champions every 20 seconds, while Gathering Storm works on everything with essentially no cooldown. So Gathering Storm makes your autos and abilities stronger, which helps you CS, take towers, dragons, barons, rift, inhibs, etc in addition to helping you kill the enemy quicker.