They are very similar. Both include an explicit patent grant. However, GPLv3 requires that redistributions keep the original HDL intact, which I prefer. Also, the fact that one of the primary authors considers it a good choice for hardware is also a factor.

Good question.

I wonder if the patent clause in GPL comes into play at all... and what constitute a 'derivative work' from HDL code...

http://www.gnu.org/licenses/gpl.html#section11