Aetherflux Reservoir has quite a few problems Tendrils doesn't.
1) It needs to be played first, which is awkward.
2) It can be destroyed and countered, which Tendrils can't.
3) It cannot be recurred with Yawghmoth's Will or Snapcaster Mage
I don't think either will be played "for value", though it's not exactly unheard of to use Tendrils as a big Fireblast.
Storm, as fun as it is for a few times before it gets a tad boring, is often quite hard to implement in a cube. Most cards necessary are ‘parasitic’, i.e, only good in that deck.
If your cube supports it, I would run both. The storm deck needs consistency and different payoffs leads to different decks/situations.
I've actually found that, if you support the right archetypes, Storm barely takes any space. Most of the cards are either widely playable or can be used for archetypes like Welder or Reanimator.
From my own cube, I would divide it like this:
Tendrils of agony - Fully parasitic, it's either in a storm or no other deck. It 's necessary for black storm to work, though, and it makes the deck more viable.
Brain Freeze - While it's surprisingly useful for self-mill, reanimator or even a control finisher, it's a 24th card in those slots so I consider it parasitic.
These are all narrow cards, but ones that will see plenty of play in other archetypes or combo decks.
Underworld Breach - This is eminently playable and a combo with Brain Freeze and LED but it can and will be used to recur cheap spells.
Yawgmoth's Will -
Brain Freeze - Surprisingly useful for self-mill and, sometimes, as a control finisher.
Thassa's Oracle - Self-Mill, some reanimator builds, can become part of a finisher package in control decks
Underworld Breach - Combo with above, useful as a late-game threat of fire to face.
Lion's Eye Diamond - Great with Welder, Emry and some discard-heavy decks
Memory Jar - Great in big mana decks, great with Welder/Emry.
Lotus Bloom - Great with Welder, can power Upheaval or other big mana plays
Paradox Engine - A big mana card, turbocharges the Earthcraft/Elfball deck.
Fastbond - Staple of the Lands archetype, great with some top of library effects, cards like Upheaval, can be easily splashed
Seething Song - While not as useful as Dark Ritual, it can push a fast Terror of the Peaks or Trumpeting Carnosaur.
Bolas's Citadel - Useful card overall, great for Welder, Tinker, big mana decks or as a control finisher.
Elemental Eruption - Quite a few decks can power this for 2-3, which is great value.
Widely playable:
etc.
