As to speed issues, the most credible benchmark I've seen so far shows MySQL to be faster up to about 7 users, after which its performance degrades much faster than postgres's. You've probably already seen the
article and
followup;
here's the summary:
My experience has been that it's generally safe to say that MySQL people are full of crap when making claims such as "MySQL is 10x faster" or "postgres doesn't support atomic transactions, either."