I would say transparency, by orders of magnitude. The two are of course not mutually exclusive. I wouldn’t be surprised if competition advocates such as left-libertarians and anarcho-capitalists would like me to believe that competition is a prerequisite for transparency. If so, I’m not buying it. If anything, competitive markets are far more contingent on market transparency than vice versa.
Certainly competition does contribute to accountability. Competitors keep each other, if not honest, at least efficient and responsive. Certainly monopoly is the antithesis of accountability.
Certainly transparency contributes to accountability. If anything, accountability is a weak version of transparency. Whenever accountability is discussed, it is necessary to ask: accountability to whom? Who is enough of a stakeholder to be entitled to which accounting report? That sort of thing. Transparency as I understand it (at least “radical transparency”) entails the release of actionable information into the public domain. Transparency is simply another word for “accountability to everyone” and is thus the ultimate facilitator of accountability.
Since competition and transparency are both conducive to accountability, I propose a tiebreaker by asking: Does competition contribute to transparency? If so, it follows (by way of transitivity) that competition contributes to accountability. I believe that “playing one’s cards close to the vest” as a business strategy is a side effect of economic competition, just as surely as state secrets are a side effect of geopolitical competition.
If the people who hate the state but love commerce ever get their way to a substantial degree, no doubt I will be more politically free than ever. I may or may not be more economically prosperous. If I work on my persuasive skills I suppose I might even be able to wrest a modicum of economic security from a freed market situation. But even in a freed market scenario that is anti-state/pro-commerce, I will be pro-Wikileaks, in the sense that if anyone in such a society decides to “leak” their employer’s trade secrets or strategic data, even in breach of “contract,” I will support such persons’ efforts to whatever extent I find myself able.