The flows are the larger oval shaped muffler that looks like most of the other aftermarket mufflers for Mustangs. They have a deep mellow tone and aren't incredibly loud. The Magnapacks are smaller cylinder shaped mufflers that are noticeably louder and have a more aggressive sound. The catback systems they come in are otherwise the same.
I really like the Magnaflows but I've never heard the Magnapacks. With that small case the 'packs are probably pretty damn loud, though.