I prefer GPL over BSD strictly for the reason that if someone makes a modified version of Arc publicly available, then we all get to see (and maybe use) the modifications.
Mysql's license is all about trying to make money from Mysql. My guess is that pg would prefer wider adoption to trying to make money. At least, initially.
I prefer GPL over BSD strictly for the reason that if someone makes a modified version of Arc publicly available, then we all get to see (and maybe use) the modifications.