Storm sphere is the better spell, without question. It's not even close.
Evard's black tentacles affects a twenty-foot square of ground. It has no effect on creatures able to fly, climb, or jump over it. It has no chance to deal any damage if you lose concentration before the enemies' turns, and a pedant might say that it has no effect if cast indoors (i.e., on floor instead of ground). Even a creature smack in the center will never have to travel more than ten feet to get out of it.
Storm sphere fills a sphere with a radius of twenty feet--more than 25,000 cubic feet, with a footprint of 1256 square feet if cast at ground level. That's more than three times the size of EBT, just in two dimensions. There is no jumping over it; even going around is pretty difficult. It has a chance to deal damage immediately, and then it lets you add a bonus-action lightning bolt on the same turn. You compare it to a fourth-level flaming sphere, but consider these differences: the lightning bolt, having an attack roll, can crit; you get advantage on the lightning bolt's attack roll if you target something within the sphere; the lightning bolt can hit enemies as far as 120 feet apart on consecutive turns, while the flaming sphere moves a maximum of thirty; and the flaming sphere doesn't come with a massive region of damage-dealing difficult terrain attached. Instead of flaming sphere, think of it as gaining up to ten turns of the sorcerer's quickened-spell metamagic, and also your cantrip does 4d6 lightning damage and sometimes gets advantage. Oh, and storm sphere also disrupts auditory perception, meaning that it can help your allies sneak up on enemies as well. Oh, and you mentioned scaling up flaming sphere; that's another thing, EBT can't be scaled up.
Here's a hypothetical simulation that I feel is pretty fair:
Evard's Black Tentacles Vs Four Creatures
Round 1: You cast the spell up to ninety feet away, end your turn. If your concentration has not been broken by the time their turn comes around, all four take dex saves. Assume half succeed and walk out of the tentacles. They attack you and your party. The other two take damage.
2(3d6) = 21 average total damage, and both become restrained.
Assume that they both use their actions to escape instead of attacking with disadvantage. One succeeds and moves out of the area. Your allies have increased odds of doing damage to the restrained enemy with attacks and dex-save spells, unless your allies went between you and the enemies--how high is your initiative usually, bladesinger?--in which case the enemy was not yet restrained on the allies' turns.
Round 2: 3d6 = 10.5 average damage to the one that is already restrained. Assume that it uses its action to try to escape and has a 50-50 chance of succeeding and walking out of the area. The other three enemies attack.
Final Result = 31.5 average total damage, four enemies attacked you five times, and your allies had increased odds of doing damage to the restrained enemy with attacks and dex-save spells. Maybe.
Storm Sphere Vs Twelve Creatures (remember, its footprint is more than three times as large)
Round 1: You cast the spell up to 150 feet away, and all the enemies within immediately roll a save. Assume half fail and take damage.
6(2d6) = 42 average total damage.
You then shoot a lightning bolt at one that is nearest to escaping and/or hurting your team. Assume, with advantage, 65.25% chance to hit, 9.75% chance to crit for double damage, and 25% chance to miss.
4d6 * 0.6525 + 8d6 * 0.0975 = 11.865 average damage.
Assume four move out of the area, one of them badly wounded by now, and attack you and your party, and the other eight have to choose between dashing to get out or making ranged attacks. Four dash, and four attack and take another save. Assume half fail, that's 2(2d6) = 14 average total damage.
Round 2:
Another lightning bolt, another 11.865 damage.
The final four move out of the radius. All twelve attack, if they are still alive, and if they are anywhere close to you, because, remember, you cast this up to 150 feet away.
Final Result = 89.73 average total damage, and twelve enemies attacked you up to sixteen times in melee and up to four at range, and your allies got increased damage from stealth. You then get eight more rounds of firing lightning bolts with an average 7.7 damage, assuming no advantage and a 45/5/50 hit/crit/miss split (which is very generous to the monsters; 60/5/35 is more typical).
. . .
Okay, okay, all that's a bit tongue in cheek and poking some fun at clutchbone's certitude. The truth of it is that both spells are very good and have their own, slightly different applications. EBT is best applied to enemies who are already engaged and thus limited in their mobility, and in the right circumstances it can give your side a huge advantage in the action economy and added damage. Storm sphere makes a fantastic bottleneck that can slow down and soften up large numbers of enemies and increase your single-target DPR at the same time. Though it doesn't matter in your bladesinger's case, it also has a lot of synergy with sorcerer features. Try them both. See which one works better for you.
