I work in a GIS shop, and I do mapping and location based stuff on the side. I am a fan of Postgres. MySQL may have some spatial components, but it's like saying that you're a pro painter cause you bought a $200 paint sprayer at Home Depot. MySQL does have cluster/replication support, but their bolt on stuff feels, well.... bolted on! Every time I use a new Postgres feature, I feel like it was actually developed. (hstore is the newest thing I'm working with)