So we have two different Least Invocations Warlocks can use to break things: Baleful Utterance(Complete Arcane), in short Shatter at will, and Hammer Blast(Complete Mage), which lets you apply your full Eldritch Blast damage to objects instead of the usual half. Problem is, I can't tell which one would be more efficient starting out as a 3rd-level Warlock, or which one would be the most useful at higher levels; anyone mind explaining the finer ups and downs of these two invocations?
Baleful Utterance is usually going to be the better choice
Baleful Utterance is going to almost always be the better choice, given that it scales at more than twice the rate Hammer Blast does (Baleful Utterance shatters an object, period, versus Hammer Blast dealing 9d6 at level 20). In addition, Baleful Utterance has other uses, including the AoE effect of Shatter as well as forcing a Fortitude save versus being dazed and deafened if you single target an object a creature is wearing or holding. The only instance in which Hammer Blast might be better is if an object was resistant or immune to Sonic damage.
If the object in question is magical, Baleful Utterance allows for a will save to negate, but Hammer Blast requires a ranged touch attack. It will be situational which is better.
Baleful Utterance is much better in the rare instance when you are fighting a crystalline creature, dealing 1d6 per caster level up to a maximum of 10d6, fortitude for half.
Typically, you don’t want to damage magical objects; they’re too valuable as loot. Shatter is therefore better, because it automatically works against the objects you don’t mind destroying: nonmagical locks and things that are giving you a hard time but aren’t worth keeping yourself.
Also, since most magical objects have pretty high Hardness, even full eldritch blast damage isn’t going to break through them with much speed. For situations like that, an adamantine pick or something is better. Or someone who knows the mountain hammer maneuver.