Is shorter dist/higher speed better than longer dist/slower speed?
 

Or is it better to alternate days of each? What have you found to be true for you? What is the conventional wisdom on this (if any)?

It depends on what you want to do.

I think if you are building your aerobic base, long dist/slower speed is best.

If you already have a good base and are training for a race, a few weeks of shorter dist/higher speed is appropriate.

Depends...is shorter time/higher speed better than longer time/slower speed in bed? It's all about what you want to accomplish :D