And only resisting fire is never, in absolutely no possible situation, better than resisting fire and optionally also lightning which makes it, always, less powerful.
Resist Fire = Resist Lightning = 1 sorcery point
Resist Fire = Careful Spell / Distant Spell / Empowered spell / Extended Spell / Subtle Spell / Twinned Cantrip = 1 Sorcery Point
Resist Fire + Resist Lighting > Resist Fire
Resist Fire + Resist Lightning = Resist Fire + careful spell or distant spell or empowered spell or extendend spell or subtle spell or twinned cantrip = resist fire + 1 sorcery point
So...
Derren said:
The mixed sorcerer has more option while the double sorcerer lacks half of a class feature.
...is untrue. The mixed sorcerer has the option to spend 1 sorcery point on a different kind of resistance if they need it. If they do that, they can't do other things with their points. They do it, and later in the day, they fry their ally with a
fireball because they can't afford Careful Spell. The double-sorcerer does not have a resistance option, which means they also don't ever spend that sorcery point there. Instead, they get fried and get to use
some other ability more often instead. So later, they avoid frying their ally because they can afford Careful Spell.
That in no way makes the double sorcerer weaker than the mixed sorcerer,
especially given the niche usefulness of an extra resistance.
Another way to say that: that part of the Elemental Affinity feature doesn't actually increase the power of the sorcerer. It simply gives them more options. And a double-dragon-sorcerer will not have as many options.
But champions aren't weaker than battle masters or wizards or clerics because they don't have as many options. They simply have fewer options.