Do posts appear on HN or Reddit first?

If it's within minutes, does it truly matter? Also, neither site really "breaks" news.

Anyway, to answer your question...

You will probably have to measure it, if you want. Should be too hard. Another, better, solution might be to generate and publish a feed that combines the two, without the redundancies. You could even include links to the separate discussions.

Reddit "breaks news first" for the vast majority of stories, simply by having a much larger userbase. I see HN as being far better filtered though, with higher quality comments and signal/noise ratio.

depends on what type of post. there's plenty of stuff that doesn't show up here or doesn't do well here that shows up on reddit and does well, and vice versa. the crowd is similar, but different.

It's definitely Reddit most of the time. It's a larger site.

If you mention wikileaks or aethism then of course reddit.