In a “perfect world” capital market, how important is a firm’s decision to pay dividends versus repurchase shares?

In a “perfect world” capital market, how important is a firm’s decision to pay dividends versus repurchase shares? Under what conditions would you have a tax preference for share repurchase rather than dividends? Would managers acting in the interests of long-term shareholders be more likely to repurchase shares if they believed the stock to be either undervalued or overvalued?