It is more power as in situations where an additional resistance is the more useful, there more powerful option, the mixed sorcerer is able to take it while the double sorcerer can not and thus is weaker in this situation.
...and it's weaker in situations where an additional resistance would be of some usefulness, but less useful than Subtle Spell (for instance). And I'm repeating myself, but balancing classes based on "use in any potential situations" is not a solid basis for measuring power in a game like D&D where the potential situations are literally limitless. So that's kind of a wash -- niche situations might prove the ability to be weaker or stronger depending on that situation
like every other ability.
This is the same logic that says having magic missile and shield is no more powerful than just having magic missile, and it just doesn't stand up to scrutiny.
More options are not more power, they're just more options. If you don't have any cause to ever actually cast the
shield spell, having it there doesn't mean anything.
In fact, specifically, if (for the purposes of discussion)
magic missile ends combat one round early and
shield negates one enemy's turn, it really doesn't matter if you only have one or the other. If critters are doing, say, 5 damage per round, then either one avoids 5 damage.
Analogy: You and your buddy can buy two things with $5, blue boxes or red boxes. Say you guys want both equally, 50% chance of buying either -- you can't say whether you're going to want them to be blue or red when you're buying them. At some point, your buddy finds a store that also sells green boxes for $5 as well, and now there's a 33% chance of him getting any one product -- we still don't know what color is going to be more useful, so there's an equal chance of any color getting chosen.
66% of the time, the green box isn't going to matter. Functionally, you both come back with a red box or a blue box, you've both spent $5, you have the exact same outcome for the exact same investment.
33% of the time, your buddy buys a green box instead.
In that case, your buddy doesn't have anything more valuable or more important or more vital than you. Remember, we can't predict the situations we're going to use this box in, we don't know up front which color is going to actually be better (if any), because there's just too many variables.. If your friend has a green box, doesn't have
more boxes than you at the end of the day. He didn't get more for his money. He doesn't have more than you. He's still bought one box for $5, he's spent the same amount and received the same amount as you have. In some small subset of the time, he just did something you couldn't do -- buy a green box.
More options isn't more power. You get the same amount of return for the same amount of investment. No one got
more out of their input than anyone else, you just got different things. Having access to that green box doesn't make your buddy any more likely to pick the most useful kind of box, since the situations are functionally infinite, any box has about the same potential utility (which is why they all cost $5).
More options IS more options, though. That's not nothing, it's just not
necessarily better. It's variety, complexity, adaptability, but it's not more powerful. 66% of the time, being able to buy a green box doesn't change the actual outcome at all. 33% of the time, the outcome changes, and that's only relevant in specific niche situations that you can't predict when you're buying the box. And heck, sometimes it's not even a GOOD relevance - if your buddy buys a green box this time and you discovered later that the blue box was more valuable, your buddy's extra option just cost him. You have no way of knowing what box is going to be more useful, because you don't know what the situation is going to be when you're choosing your box.
Options aren't nothing, but they
aren't power, and if you are concerned about a specific build's lack of
options, that's a different conversation than their lack of
power.