I disagree to a point with the idea that mySQL will be slower with small systems. mySQL is optimized for data storage and retrieval. It'll only be slow if you put extraneous strain on the system. So storing images in a database is extra strain that will slow it down; however, if you're just keeping text, it makes sense.
Text-files can be tricky, only if you're not positive about what you're doing. The reason most Blogging software is written with a database backend (as opposed to a text-file backend) is because databases are designed to store information. Text files are more on the side of sharing information in a global way (i.e. any system can open a .txt file and see its contents).
Ultimately the choice is yours; however, mySQL is not complicated if you sit down and think out what you want, and how you're going to use the information. I.e. don't use a column to define a "category", and then insert the category name into the column (so you have 300 rows of the same value) only to find out that you messed up in typing. Now you have to go back and change 300 rows, when you could have referenced it to an external table in the database and changed it once. Text-files are much the same way. You have to change everything almost by hand.
I'd say if you're going to use text files, just take it one step further and use XML since that's more of a structured thing than text files. And PHP 5 integrates with XML really well now. PHP 4 has an extension that can be used, or you can create your own functions to read the XML....
So really, mySQL adds things that text-files don't have (like indexing for searches, and less-space). And text files have something SQL doesnt: portability. So it's really a question of:
-- Do I need to move the information around from area to area, or can I store it in a structured database, and when I need to change servers, just export to an SQL file and import later?
I find that mySQL is better than text files in most cases; however, there are some instances where text-files will be better. Notice I didn't say faster....