Difference between dft and the Spectrum

Dear Sir, 

I would like to ask you please what is the difference between the dft function from the calculator and the Spectrum tool provided by ADE, 

I want to simulate the THD and the SNR of my amplifier, which one is better to use, 

Some people claimed that Spectrum tool is not that accurate, I would say it is up to their setting, what is the best setting to increase the accuracy of the Spectrum tool to read the THD and the ENOB.

Thank you in advance

Best Regrds