Unlike Postgresql, MySQL 5.1 seems to have all the GIS column types inbuilt (though I’ve not used them, so there might be less there than meets the eye). As far as I’m aware, MySQL is also the more strongly supported of the two.
I could understand if the switch had gone the other way, or from MySQL to MariaDB if the issue is keeping it non-commercial, but going from MySQL to Postgresql is a puzzle to me. If it’s already been explained somewhere, could someone give me a pointer?
(I suppose I should declare my interest: I understand MySQL pretty well already, but Postgresql not at all yet, so it would have been more convenient for me if MySQL were still the system of choice. I know: selfish selfish