FSA vs. HSA? When?

The topic was sparked from another thread on investment prioritization found here: https://www.reddit.com/r/Bogleheads/comments/1js7hna/prioritizing_investments/

When does it make sense to establish an FSA vs. HSA?

I'll agree ahead of time it depends on the plans offered (I'm sure a nicely structured high deductible plan funded by the government is going to be better than one in the private sector). But are there are rules of thumb or key data points that can help one decide between the two?