Whether it is better to buy national debt or bond fund depends on your risk tolerance and income expectation.
National debt: the income may be less than that of bond funds.
Bond fund: the income may be greater than the national debt, but the risk is higher than the national debt. Some bond funds sometimes lose your principal.
Treasury bonds are safe, but the amount of funds needed to buy them is very large; Bond-type bond funds will have higher returns, based on the net value of bond funds, and require less funds.