Your question mostly depends on what Mac you have. If your Mac has Line Level input, then you have to have some kind of pre-amp to get a signal into your Mac. If your Mac has Mic Level input, then you could use one of the "Garage Band Cords". I assume you're referring to Monster Cables product.
Then it depends on your objectives. If you're serious about recording, then you're going to want a pre-amp. I use the M-Audio Fast Track Pro because I need phantom power for my condenser mics, I usually need to record two tracks simultaneously also.
I would recommend the Fast Track Pro because it works great with Garage Band, but it all depends on your objectives.