Without a doubt, go for Postgres.
PostGres+PostGIS is an fully featured spatial database, and has a lot of documentation, and you'll easily find help from people on forums and here.
MySQL was late to the spatial field, and lacks many features which are there in Postgre+PostGIS. Even the community using MySQL for spatial purposes is minuscule when compared to the users of PostGIS. The future of MySql is also question, with most of the Original Developers having forked the code, and moved to a different project called MariaDB.
Fore more details, have a look at these two questions: PostgreSQL vs. MySQL - Advantages / Disadvantages with a spatial component & MySQL vs PostgreSQL spatial application