This is simply illogical. A mixed sorcerer does not spend points on lightning resistance on a gamble that it might come in useful, he spends it when he knows that it is useful and in that case he is superior to the double sorcerer. And if that situation never comes up, he is still equal. In no circumstance is the mixed sorcerer weaker than the double sorcerer.
Actually, at this point is when you've become illogical.
Have you looked up the number of electricity-using creatures in 5E? I have; there's a distinct lack of ones where simply activating it would give a distinct advantage (note that this is different from fire). Most electricity damage is going to come from spellcasters, for whom no sorcerer can know the spell list of without capacity to tell the future. Notice, also, there's a distinct lack of spells that would give enough future information to make that plausible.
As such, a sorcerer will almost never know lightning resistance would be useful until after it has proven useful.
Note that is different from fire damage, which comes up so much that having it on as often as possible is simply being practical.
According to your logic a sorcerer with a single known 1st level spell, magic missile, is better than a sorcerer which knows magic missile and shield because he does not "waste" spell slots on defensive spells. Same example with the Battle Master which I would like you to answer instead of dodging.
Nice strawman.
Spells require different considerations than class traits. First, what is there in options that is actually better than a defensive spell for filling that slot? Realistically, not necessarily enough. And not taking a second spell is sacrificing power, especially one that is useful because its reaction time casting means that it always applies up until it is actually cast (as long as you slots open to cast spells), after which it stops applying. That's part of why the PHB recommends it.
As for the battle master: I gave you an answer. I later edited it out because I felt I was putting in too much information and that you'd not read the whole quote and I'd have to give it to you again, but you managed to quote me anyway. In any case, if you want your answer, scroll up to where you quoted what I said about the battle master and reread it. The answer is there.
Your reasoning is flawed in several ways. You think that lightning resistance gets always used without knowing if it is useful and you think that more offensive options are always useful. Both is wrong.
And your reasoning is flawed in a number of ways. You assume sorcerers are prescient, you believe that options equal power, you do not understand how someone can have a tactical pattern outside of your worldview, you seem to assume that choosing to give up some options related to defense automatically means giving up all defensive options, you don't even bother to read all of what you quote, and you automatically assume that a person must have the tactical style of thinking they are discussing without bothering to even ask if it is their viewpoint.
Now, are we done launching ad hominems at each other?
