What is the difference between 6.x-1.x-dev and 6.x-1.0-alpha1?

at this point they're identical. but as soon as commits are made to HEAD the -dev will automatically be updated to include those. i created the -alpha1 at a relatively stable point when i think most things are working so it's a "safe" version. the -dev is kind of a grab back, you never know quite what you could get from it, maybe new bugs, or maybe new bug fixes. so unless you feel comfortable debugging i'd suggest using the offical release rather than the snapshot. you might find http://drupal.org/node/197584 interesting reading.