Jonathan Ellis, a developer and Apache Cassandra project leader, writes about distributed databases, Cassandra releases, and data structures like bloom filters.
HN commenters link his bloom filter write-up as a reference for hashing and Cassandra's internals, and his Cassandra posts for project plans and database comparisons.
First off - whatever you're doing, you'll probably be fine using either.
That said, I'm generally more frustrated when using MySQL than when using PG. Here's a sample of the problems I've encountered from using both MySQL and PG. I haven't updated my list in a while now - please feel free to correct me on things - but hopefully it's a little more illustrative than that Wikipedia feature matrix, and a little more specific to MySQL vs. PG. (This list is a cleaned-up selection from my notes wiki at http://yz.mit.edu/notes/Hackery.)
No referential integrity.
No constraints (CHECK).
No sort merge join, let alone hash-join. http://www.dbms2.com/2008/07/10/how-is-mysqls-join-performance-these-days/, http://www.mysqlperformanceblog.com/2006/06/09/why-mysql-could-be-slow-with-large-tables/
Generally poor at analytical workloads, since it's designed for transactional workloads.
Can't reopen TEMP table - WTF? (Still not fixed!) http://bugs.mysql.com/bug.php?id=10327
Multiple storage engines has always restricted progress: http://www.mysqlperformanceblog.com/2010/05/08/the-doom-of-multiple-storage-engines/ (PG also supported multiple storage engines in 80s, then concentrated on one)
No WITH clause: http://stackoverflow.com/questions/324935/mysql-with-clause
Crappy errors: “Incorrect key file for table ‘stock’; try to repair it” on “alter table stock add constraint pk_stock primary key (s_w_id, s_i_id);” where stock is in InnoDB (which has no “repair table”) means I have no…
This happens to be an oldie but a goody. On the same topic, I might as well spread the wealth of links I've amassed for learning and understanding Cassandra:
Up and Running With Cassandra
http://blog.evanweaver.com/articles/2009/07/06/up-and-running-with-cassandra/
Building A Small Cassandra Cluster
http://posulliv.com/?p=149
Cassandra PHP CRUD Class (Pandra)
http://github.com/mjpearson/Pandra
Getting Started - CassandraWiki
http://wiki.apache.org/cassandra/GettingStarted
Cassandra In Action
http://spyced.blogspot.com/2010/03/cassandra-in-action.html
Cassandra is hugely biased towards making writes fast [1] and there's no indication in either Digg article that they (somehow) changed that fact in their deployment by tuning. Digg loads the complexity during the write by denormalizing but that load is on the application server not the data store.
[1] http://spyced.blogspot.com/2010/01/cassandra-05.html
You did in fact say web2py won:
"web2py is the only framework that participated to the www.flourishconf.com rumble (develop a web app in 24 hours) and won because all the other frameworks chickened out."
http://spyced.blogspot.com/2008/04/m-half-baked-thoughts-on-python-web.html?showComment=1207575720000#c7863386267780636294
related: cassandra's use of bloom filters
http://spyced.blogspot.com/2009/01/all-you-ever-wanted-to-know-about.html
and also related: murmurhash, a hash which is well-suited for bloom filters and that is much faster than SHA-1
http://murmurhash.googlepages.com/
He missed the best way to generate hashes. Most people do: http://spyced.blogspot.com/2009/01/all-you-ever-wanted-to-know-about.html