I'd like to know the proper method to solve the problem below:

We have acquired new furniture for the office. The invoice for $6,000 offers two ways to pay: we can pay the entire amount by September 1, or we can pay $3,060 by September 1 and $3,000 by January 1. How does our decision depend on the interest rate at which we can invest our funds?

I've come up with the following two methods, one an Excel formula and the other a simple interest formula but they give two answer. Which one is actually correct?

Excel: =RATE(3000/12,0,2940,-3000) = 6.25%

Simple Interest Calculator: (1/(4/12)) * ((3000/2940)-1) * 12 = 6.12%