i am not a photoshop geek but, adobe just try to make the things simpler and simple, the tricks which take hours adobe make these things in the couple of seconds like for example the selection techniques and many more, PERSONAL PREFERENCE IS ADOBE CS5
CS4 is easier. I think I like it better. CS5 is hard. I have the extended too. Its really hard. Gonna be a while before I figure out the 3D and motion stuff.
It depends on what you are trying to do, but if you are starting to learn you can use either one. If you want to buy them, of course buy CS5, it has more features that you could use later on.