The two trees (Havoc and Conflagration) are pretty different. The similarities are very superficial.
Similarities: both of the specializations have a main spell (Fire of Gehenna for Conflagration, Shockstrike for Havoc), a fast insta-cast spell (Hellfire Stream for Conflagration, Shock for Havoc; note that Hellfire Stream is insta only if you feat Combustion, which is really an essential feat), and a Big Aoe spell (Inferno of Amher / Shockblast).
In practice each of these spell is different from the equivalent in the opposite tree: for example, Shockstrike is generally favourable for AoE (Shockstrike AoE feats from Havoc tree for example are a must for my PvP playstile), while Fire of Gehenna can provide you better single target DPS; Shockblast feated performs way higher damage then Inferno of Amher, but Inferno of Amher can apply Incinerate and a DPS debuff on targets; Hellfire Streams can apply Incinerate etc.
As you see, it's wrong to consider the two trees specular, similarities are just "aesthetical".
To continue, the two trees provide different CCs: Conflagration provides the awesome Detonation (insta AoE Knockback), while Havoc an additional long duration stun and an useful single target knockback (even if not as great as Detonation).
Remember that demo also has an innate root and stun.
Each tree also has specific spells: Waves of Flame is weak without Ring of Fire perk, but wuth that perk becomes one of the essential DPS demo spells.
Havoc provides Shocklance, which is useful (stacked 5x is an additional insta cast, and everything insta is awesone..).
End trees are very different (but pretty much the most worth one, if not the only one, is Pact of the Earthwalking).
And so on..
Let's go to the point: most efficient builds are Hybrid (havoc + conflagration).
Someone goes for trybrid, adding General for Hate Decrease in PvE, or Purge in PvP (I suggest you to discard this last option for PvP, though I have tried it extensively).
But you will find a lot of discussions already about build.. So just have fun reading this section of the forum