What I did is to record the same midi section on different sound samples, like soundfont, giga, DXi, Roland SC-8850, XV-5050, etc.
Load all the recorded wave files into Wavelab, then perform a frequency analysis. You will SEE the difference in graphics -- it helps you to make decisions.
However, sound has to be compatible with the style of music you are making. DXi is a good choice in jazzy and easy listenning music, but not classical style. I use giga to make orchestral pieces.
I do think 肘子 is right. A good music is a good music because of the composition; hardware/software technology comes second.
