While the comparisons are nicely presented by Scott, I have to agree with appaccelerator on key points about flash vs. HDD. If you just compare Moore's law to Kreider's law (which defines the capacity growth of HDD), there isn't much doubt that in the end, semiconductors will win this fight. Just like tape, there will always be a few applications where flash doesn't make sense.

Here's a link to a blog by Pure Storage's CEO that explains how to understand what happens with performance on hybrid arrays:
http://www.purestorage.com/blog/the-risk-of-over-promising-and-under-delivering-with-hybrid-storage-arrays/

Appreciate not every application in the world "needs" the performance of flash, but if you can get it for the same price or close, why would you choose HDD?