,Neither. The prefatory clause and the operative clause are independent in terms of meaning.
However, the historical record speaks much more supportively to the first assertion rather than the second. Opposition to standing armies as the professional coercive force of the government is well established in the historical record. A citizen "militia" made up from the adult population was the solution, so viewing the prefatory clause as a way to counter the militia has no basis.
"A well regulated militia being necessary to the security of a free state, the right of the people to keep and bear arms shall not be infringed."
There are two things going on in the prefatory clause. First, a recognition in light of [then] recent events, ie the War of Independence, that in order for the state (by which, read "nation") to remain free, it must have a force capable of defending against aggressors - thus the call for a "well-regulated", ie effective, militia. Second, also in light of [then] recent events, the opposition to a standing army, such as that of the British, (don't forget that the 3rd Amendment forbids the garrisoning of troops on the people!) and the view that it should be a citizen militia that ensures the security of the state. So, the prefatory clause states a need for an armed force to protect the state from aggression, and states that in order to ensure that the state remains free, that armed force ought be made up from a citizens' militia. The operative clause, which states a course of action and law, forbids the state from infringing on the unalienable right of the people to keep and bear arms. An armed adult populace being the source of the militia and, by virtue of being armed, guaranteeing the security of a free state both, as and when required to form an armed force against enemies foreign, and as a counter to the potential tyranny of enemies [of freedom] domestic.
The Second Amendment should not be viewed in isolation. The First acknowledges that the government [the state] may not control or regulate religious belief, speech/expression [and implicit is 'criticism of government'], or peaceable assembly. The Second ensures these unalienable [neither granted by government, nor subject to removal by government] rights by virtue of an armed populace capable of throwing off tyranny (of ensuring the security of a free state), and the Third addresses a major grievance among British colonists, as well as further guarding against a standing army capable of enforcing tyranny: "No Soldier shall, in time of peace be quartered in any house, without the consent of the Owner, nor in time of war, but in a manner to be prescribed by law."
So, the prefatory clause addresses the well established opposition to a standing army as a potential coercive tool of government while acknowledging the need for an armed force to address aggression. A well regulated militia is necessary because the state will need to defend against external aggression. And, a militia should serve this purpose because a citizen force, as opposed to a standing professional army, is less likely to be misused by the state as a tool of coercion, thereby ensuring the security (against enemies foreign and domestic) of a free (from foreign domination and domestic tyranny) state. The operative clause forbids the government from acting to deny the populace their unalienable right to self defense (from both foreign domination and domestic tyranny) through owning and carrying firearms.
Anti-civil rights campaigners have used the prefatory clause in various ways to attempt to undermine or reinterpret the operative clause. The most old and tired effort is to link the prefatory and operative clauses, ie only members of a well-regulated militia (the National Guard?) shall have the right to keep and bear arms, or only in service to the militia may one keep and bear arms. These tawdry efforts have been so well refuted elsewhere, that I will not rehash the case here. A more nuanced effort to attack the unalienable right of the individual to keep and bear arms is to suggest that because the prefatory clause has no force of law, neither should the out of date operative clause. Here, the anti-civil rights campaigner notes that the US does have and does maintain a standing army (and Navy, etc.). Therefore, they argue, since we have de facto ignored the prefatory clause, we can safely ignore the operative clause. However nuanced, this attempt is bogus. The prefatory clause states an opinion, it creates only "a sense of the Founders". It does not create law. The prefatory clause does not direct action, nor does it forbid action. If it read "A well regulated militia being necessary to the security of a free state, the creation of a standing army is forbidden, and the right of the people to keep and bear arms shall not be infringed.", then perhaps this gambit would have merit. In this case, the prefatory clause directs a course of action and law by forbidding the creation of a standing army. And, were it so written, if we now had a standing army, one could argue both the illegality of that army and that the right of the people to keep and bear arms may indeed be infringed. But it wasn't written that way.
So, the operative clause is the matter of substance in the Second Amendment. It directs action and law. It forbids government from acting to contravene the unalienable right of the people to keep and bear arms. Whether ANY regulation or restriction of that right constitutes that right being "infringed" is another matter and another discussion. The prefatory clause does not direct action or law. It provides a sense of the Founders that while an armed force may be needed, it ought to be established as and when necessary by a militia. Since the opposition to standing armies is well established in the historical record and since the Third Amendment to the Constitution addresses an important grievance relative to standing armies, this is well established fact. The "militia" is also well defined in both the historical record and subsequent US law as an armed body legally constituted from the adult population. There is no basis, other than speculation, to suggest that the operative clause is designed to counter the militia, as the militia is constituted from that body of people whose right to keep and bear arms is proscribed from being "infringed" upon.