I would agree much about PG with StarShip3000 said.
About MySQL, I think it's too much dooming speech about Orcle owning it. It is still remain being developed as Open Source. Maybe they sell a commercial version, but they already did it while as under MySQL AB.
Speculations apart, I have some points to bring about MySQL. As it has some nice feats, has equally bad points.
For example, MySQL is most know for being a super duper fast RDBMS. And it is. When tables are using MyISAM engine. What this means? Means that even a simple requirement as foreign keys consistency will not be available. Then you can imagine about all the rest.
If you use InnoDB engine for MySQL tables, then you can count with FK, safe transactions, but a performance not as fast (pretty in the same level as PostgreSQL, which is good). Ah, and you cannot count with full-text search feature for example.
MySQL is this: or you have performance, or have a decently ruled database, but never both.
Postgre, which is my choice, is fully featured. Does not have the MyISAM tables performance, but is fast enough.
Most people does not know, but Postgre is even more full featured than you can imagine. Let me tell you guys about a nice thing on PG. It counts will a genetic algorithm on its query parser.
Query parser is the piece who parse the query and decide what plan should follow to extract data you asked. When it parses a query, it can achieve some different ways to extract same data. It picks the most probably fast, and execute. But it executes other plans too sometimes, to check if it is good or not indeed. And logs stats about those runs.
Along the time, it knows, through statistics, what is the fastest plan to run, so it gets faster on its own. How much you use it, faster it gets. If tables changes, this process allows it to optimize itself.
OK, I've talked too much.
Well, I truly does not know M$ Express Server, so, I can't talk about it. But Now I think you have a better panorama about Postgre and MySQL.
Cheers.