#Which is better?
dark void
The first one in this set I feel is better (but the third could potentially work—centered text is an aesthetic choice). For optimal readability, the ideal line length for body text is 45–75 characters per line, including spaces (http://webtypography.net/2.1.2).
Like I said the third version would also fit this rule; however, I'm not as partial to centered text as it tends to highlight ragged lines more, making the layout feel less clean than when left aligned. Hope that helps.
dark void
This is hard to determine as it's only a piece of the site, so not knowing what the rest of the text looks like (fonts and styles used) it's hard to determine which is a better fit.
With that said, the serif font feels cleaner and more sophisticated. You might want to even out the space between lines though (there's a lot of space between lines in the larger text but very minimal space after it, before the smaller text).