Garageband 4 has more than GB 3. this is true. But, Logic contains all that GB 4 contains and more.
In fact, when you install iLife 08 you should select the option not to install the loops as you won't need them, I think.
well, you can make a podcast using iMovie or iPhoto and then bring in the movie or slideshow into Logic and create a soundtrack and/or voice over for it there...
However, Garageband's podcasting feature makes it very easy.
It all depends what you want to use it for. There are many artists who use Garageband to write sketches or ideas for their songs and albums... but to mix i would go to Logic every time.
The effects are better, Logic has powerful and more precise Mixer windows. Garageband is great, but is lacking all the Pro mixing features like Buses and Quality reverbs etc!