I personally use MySQLi.
The only differences that I know between the two is how you bind params during a query, other than that, I don't know anything major MySQL wise. However, PDO supports more than just MySQL, so that's a plus for people who use something else, but that's all I can think of.
Personally I think both are pretty good, although this can give you an idea. Extracted fromYou must be registered for see links
So here basically you should go for MySQLi in my opinion, but a plus for PDO is that you can use it for almost any database driver ( Oracle, SQLite, MySQL, Ms SQL, PostGre, IBM ) which is pretty cool and neat.
So, it's up to you to decide - You can always go hardcore and use both ( So awesome, do it for Rev3 can explain you how on MSN )