Pick the 4.1 set. It allegedly has better SNR and probably better bass, as far as you can call a 40 Hz limit "good".
A center speaker won't make much of a difference to you, and the 5.1 set especially isn't worth it if it's more expensive.
Center speakers are best for balancing out the sound in home theatre arrangements especially when the speakers are all far apart.
And kvndoom, you're on the wrong path to redemption, you do NOT use center speakers for music. Ever. You just don't do that.
Too lazy to change my settings every time i switch to doing something else with the computer. Once I actually get a good center speaker (may take some time and money for that...) I'll do some objective listening and decide what I really like the most. My Audigy2 with kx drivers allowed me a lot more flexibility than I have now, and I had my setup tweaked to (my) perfection, as far as tonal balance and imaging. So even though the Xonar possesses vastly superior sound quality, I'm having to learn how
to listen to stuff all over again.
Granted, Dolby Headphone mode is freaking incredible. The one time I do bother to change modes is when I put my cans on.