First off, welcome to Mac's! I also have a MBP, and used Garage Band initially (love it!) and have recently moved to Logic Pro (love it even more!). The loops are interchangeable between them, yes. Those that aren't can be converted in a program that comes with your MBP, called Apple Loop Utility.
Since you'll be using a MBP, you'll have a firewire 800 port, and can purchase an external hard drive at this speed in order to store your loop files. If you get a good drive (I recommend a G-tech), you can utilize them in real time within the programs. Another (better) option, is to get an expresscard SATA port, and get an eSATA drive for the same purpose. This way you don't overwhelm your main hard drive with loop collections, and can organize them by use in folders, for quick searching/locating.
Both programs come with loop collections, and Logic Pro will keep you busy for a while, without purchasing any 3rd party loops. Once you reach the point of needing more, or find you need something more specific to a project, you can buy loop collections at several online music stores. I'll link you to some I've used:
Sweetwater
Musicans Friend
That's just a drop in the bucket of what's out there. There are also sites that allow free downloads of loops, and those that have fee based downloading. On top of that, you can create your own loops. I suggest getting a midi keyboard that's usb bus powered, as well as the option to plug into the wall, and experiment, and have fun. You can record in both programs as well. I wish you the best, and know you'll be astonished by the abilities of both GarageBand and Logic Pro. BTW, here is a link to the
thread for discussing topics related to music creation on macs. You can get more information there as well.
🙂
