Why does Heroku use Postgresql?
Postgres is better than mysql in many ways. You can read these posts about migration rails app from mysql to postgres. Though mysql is more popular than postgres, but instagram is using postgres maybe due to these reasons. I feel postgres is much more mature and robust than mysql. Whereas mysql is more easy and simple to use.
Migrating MySQL to PostgreSQL in Rails « m i n d l e v
Converting Rails application data from MySQL to PostgreSQL
For data migration, a handy little script: Rake task to transfer a Rails database, say from MySQL to Postgres and back again
Compare the length of this list of MySQL gotchas to this list of postgresql gotchas. MySQL is much more likely to mess you up.
Generally speaking I find pgsql lends itself to 24/7 operations better than mysql. Also, it seems to have fewer footguns build into it. I think if you're more familiar with one or the other, that's more likely to influence your decision than anything else.