I'm creating a Lizardfolk, Draconic Bloodline Sorcerer. And I was thinking that the dragons that would fit him best would either be Black or Green dragons. Since they live in or near a swamp (and that's where Lizardfolk generally live). Which would you choose based on how many/effective spells there are of the respective damage types (Acid or Poison). Also, Poison isn't included in the Elemental Adept feat for some reason.

Thanks for your input!

Poison isn't included in Elemental Adept because it isn't an elemental damage type. For which type you choose, honestly, it depends on your campaign. Poison damage is more common, but it's also more commonly resisted. Acid damage is rarer, but is almost never resisted.

Acid damage is dramatically more useful than poison.  Especially if you've got Tasha's Guide and can thus take Tasha's Caustic Brew.

Looking at past D&D lore for ideas, the Blackscales are a subrace of Lizardfolk from 3e that claim to be descended from Black dragons, while the Poison Dusk Sub-species of Lizardfolk from the same edition were more like the Green Dragons (Though I don't know if it was ever speculated in lore that these Lizardfolk came from breeding with Green Dragons). So thematics wise either option is very sound for a Lizardfolk to get their draconic origin from. 

If I were to choose however, I'd go with Acid for damage as fewer things are resisted/immune to it. Acid Splash (cantrip), Chromatic Orb(1st level), Tasha's Caustic Brew (1st level), Dragon's Breath (2nd level), and Vitriolic Sphere (4th level) are admittedly the only spells that do acid damage. However, with the Transmuted Spell metamagic option you can turn a spell's damage type to acid as well, which can open up some options for you.