If you look on the Apple website, you'll see testimonies from a lot of professional musicians who use Garageband for recording songs as they're composing - mixing together guitar, vocal and keyboards and engineering the mix. GB is a great editor for just laying out tracks and loops quickly, and it has some nice effects and options. If you just want to experiment with pre-recorded loops and add some vocals, it could be all you need.
The one thing GB lacks is accuracy. The editors lack the precision for adjusting/editing audio, and it can be frustrating trying to tweak tracks for timing. It also suffers from high latency when recording audio over USB.
Logic Express is a great package and offers all of the accuracy you'll need, and you'll almost certainly never find something you want to do that it doesn't support. As I record audio rather than working with samples, the extra price for Logic Studio is a waste of time, but the loop libraries it comes with could be worth it for you. In terms of the core packages - Logic Express 8 vs Logic Pro 8 - I've yet to find a difference, and suspect there isn't one.
Try out Garageband and see if it works for the music you want to create, and if it does, you can save a lot of cash. If it doesn't (I could see the limitations GB was applying to me from the first time I tried mixing a few audio tracks) look at the two Logic products and see which is going to work for you/your wallet.
Hope that helps.
BR