As a one-shot bad guy, warlocks are woefully underpowered. A level 6 warlock is going to be doing 3d6 damage per eldritch blast. That is an average of 10.5 damage per hit, which requires a ranged touch attack to hit (not always easy at level 6 with a BAB of +4). Plus, the warlock is only hitting one opponent per round at that level.
A level 6 sorcerer is going to be able to cast at least 3 fireballs per day, (most likely 4 if they have a decent CHA) each doing 6d6 damage. That is an average of 21 points of damage per round. Yes, you can save for half and get the same 10.5 the warlock gets with no save - but, the fireball can affect many targets with its 20 foot radius area of effect.
After that, the sorcerer can crank off at least 5 scorching rays, each getting 2 rays at that level and doing 4d6 damage with no save. That is an average of 28 points of damage per round, if the sorcerer hits twice. (again, no given)
Then, the sorcerer can crank off at least 6 or 7 magic missiles. Each round, he would then do another 3d4+3 damage, or 10.5 per round.
So, the sorcerer has put out way more damage over 14-17 rounds of combat than a warlock of the same level. And, the times where his damage is equal, he can affect multiple targets (fireball with made save) or he has no miss chance (magic missile)