Prefer the 6.1 percent tax-exempt investment.
Let's do the math and see why the tax-exempt investment is the better choice. For the 8.1% taxable investment, you get taxed at the rate of 28%. Which means that you only get to keep 100%-28% = 72% of your gains. So 0.72 * 8.1 = 5.832 which means your effective earning percentage is only 5.832% which is less than the 6.1% rate you get for the tax-exempt investment. Another consideration that wasn't taken into account for the question is the earnings on the taxable investment may push you up into a higher tax bracket. Which in turn increases the tax burden on your other investments. So the better choice here is the 6.1% tax-exempt investment even though that first glance the 8.1% investment looks higher.
