And more HD = worse buyoff, which is a consideration.
Which is a point of why Steel is so awesome. The Steel is an exception.
btw correction to my self, it's the mist dragon that has the gaseous form. My bad.Loredrake Mercury Dragon Sorcerer 1: ECL 6 (4hd, 2la). Casts as a 3rd level caster.
3d10+1d4 (23.5 hp), 3/3/5 saves, 38 skill points, +2 con & +4 dex.
Fly 200 (perfect), takes +50% from cold spells, is tiny (-8 to most opposed checks).
Immune to stuff that would blind or dazzle it and +3 on saves vs light or pattern effects and immunity to fire.
Loredrake Steel Dragon: ECL 6 (4hd, 2la). Casts as a 3rd level caster.
4d10 (26.5 hp), 4/4/4 saves, 42 skill points, +2 con & cha.
Fly 150 (average, no hover), is small (only -4 to most opposed checks).
+10 save bonus against poison, immune to acid, spell resistance 16, 5/day change into any animal/humanoid with no duration.
The Steel has a hair bit more hp/save/skill points than the Mercury since it didn't have to take that level in sorcerer to achieve the same spellcasting ability. No elemental deals extra damage to it and it's already packing spell resistance equal to it's ECL+10, this will be out scaled but for now it's nice. It has a racial bonus to it's primary casting stat too which is always better than +4 in a secondary stat.
The icing on the cake is probably it's alternate form ability is polymorph only limited to humanoids and animals. Steel dragons are already masters of disguise without a single rank in the skill and are capable of trading their flight for even higher physical abilities. Mercury won't have alter Self till next level and thats capped at 5HD so it has to wait until ECL 11 to get around it, by then the Steel was changing into 10HD creatures a level ago.
Finally, consider that paying a druid 10,450gp for Mantle Of The Icy Soul & 11,360gp for Mantle Of The Fiery Spirit granting both dragons full immunity to both fire & cold. In which case the steel dragon's acid immunity truly pulls out ahead as it is an immunity you cannot simply buy with chump change later on.
And so I consider the Steel Dragon in every way superior to the CharOp favored Mercury Dragon. In open spaces flying in a circle works every bit as good as the ability to hover and 150ft is plenty to out distance virtually any land creature so snipping 50ft of speed is perfectly practical.
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The OA dragons lack LA entries in the OA book. Perhaps another book updated them kinda like Dragon's of Faerun added LA to some of the 3E Faerunian dragons?