Flash questions

I have the 600, I believe the big difference is the amount of power, but I could be wrong.  The 600 puts out enough light for what I use my flash for.  The swiveling is a must!!!  I hardly ever shoot anything with the flash directly pointing at them.  It operates nicely, one button swivels and turns all directions, which is not how my Olympus flash works.
Compensation is the same as exposure compensation you can just dial it up or down, you are right it's a different way of doing something you can do another way.  The nice thing is that you can use the auto functions and compensation together.  Let's say it's set up to auto TTL, you shoot something and notice it is blown, just dial down the compensation a bit and reshoot.  Of course that's not the most anally-technical way to get your exposure right, but it's an option.  KNOW YOUR LCD SCEEN if you are going to do that.  Mine shows up a touch darker on my Nikon than what it will be on my computer screen and my Olympus is a touch lighter.  So experiment A LOT before you use that to test your exposure.

Anyway, I hope that helps.  Get a bracket and synch cord as soon as you can afford it, helps with red eye if you have to use your flash pointed at your subject.

Bob's right about the "commander mode" although I didn't know it was called that.  Keep in mind it's not 100% reliable... some funny things can throw it off, you know, like the sun The darker the ambient light the better.