Some have understood God to work in every agent in such a way that no created power has any effect in things, but that God alone is the ultimate cause of everything wrought; for instance, that it is not fire that gives heat, but God in the fire, and so forth. But this is impossible.
First, because the order of cause and effect would be taken away from created things: and this would imply lack of power in the Creator: for it is due to the power of the cause, that it bestows active power on its effect.
Thus, according to these philosophers, every thing is full of God. Not content with the principle, that nothing exists but by his will, that nothing possesses any power but by his concession: they rob nature, and all created beings, of every power, in order to render their dependence on the Deity still more sensible and immediate. They consider not that, by this theory, they diminish, instead of magnifying, the grandeur of those attributes, which they affect so much to celebrate. It argues surely more power in the Deity to delegate a certain degree of power to inferior creatures than to produce every thing by his own immediate volition.
These two passages give some sense of the importance in distinguishing hypothetical and categorical proposals of a particular type of argument. The arguments are in one sense very similar. There are some minor differences, due to different contexts (Aquinas is arguing against al-Ghazali, Hume is arguing against Malebranche) that would need to be considered in a full interpretation, but we can abstract from these for our purposes here. They both make the same basic criticism of occasionalism: that, while it is trying to exalt divine power, it really detracts from it, because giving power to the effect is more indicative of power in the cause. But there is another sense in which they are very different arguments. Aquinas's argument is a categorical argument: he is committed to each step of the argument, and thus the argument is an argument to a conclusion he thinks true, on the basis of premises he thinks true. It is an argument for what everyone should believe. Not so Hume's. We don't quite know Hume's actual position on these matters, but it is clear from the context that Hume is not committed to anything said in this argument. His argument is hypothetical: he may or may not believe any particular element of the argument, but his point is that occasionalists should believe this, or at least consider it more plausible than what they do believe. That is, it's not an attempt to lay out the serious philosophical reason for rejecting occasionalism (indeed, he goes on to offer what he calls "a more philosophical confutation of this theory" -- it is difficult to imagine Aquinas doing that, since on Aquinas's view the argument is as philosophical a confutation as could possibly be); it's an attempt to give occasionalists pause, to force them to ask themselves whether they are really being consistent. So very similar arguments, structurally and thematically speaking: but very different arguments, functionally speaking.
It is clear that any type of argument may be put forward either categorically or hypothetically. It is also clear, I would suggest, that categorically proposed arguments are stronger arguments than their hypothetically proposed counterparts; but hypothetically proposed arguments are safer, in the sense of committing the arguer to less, than their categorically proposed counterparts.
Thus in interpreting an argument we must consider not merely the concepts and logical structures involved; we must consider also how the argument is being put forward.