To put it bluntly after managing both for years, Postgres doesn't scare the shit out of me like MySQL does. I've had quite a few moments with MySQL doing stupid things that aren't intuitive or right, particularly in the backup/restore space. For example backup one schema and restore onto later versions doesn't always work.
Postgres has the same feel as *BSD i.e. deterministic. It is well documented and does exactly what the manual says and is devoid of surprises. It feels engineered and I can provide reliably engineered solutions because of this.
Basically I can sleep at night.
I really give less of a crap about scalability. I'd throw a bigger box at the problem or use heavy caching. I've built much larger ecommerce solutions with orders of magnitude more hits/orders than sparkfun on much smaller kit