Great question (and here comes a great answer):
It depends on your own outlook/vibe, I think.
Macworld is more serious and plays it straight (think Time or Newsweek). MacAddict is more irreverent, freewheeling and playful (think Highlights)
But seriously, that's the basic gist. I'm a Macworld guy. I used to by MacAddict, but it always struck me as a bit South Park/Beavis...it skews younger, I do believe.
Honestly, here's your best bet for Mac magazines: those British ones like iCreate (if it's still around) and Mac Creative and several others. They cost a bit more, but compared to the actual CONTENT of the American mags, they're loaded! True, multi-page reviews, tutorials and articles.
Years ago, in the mid-late 90's, Macworld was quite thick, with smaller type and truly "packed in the goods": they're reviews were long, their articles about Macs and software spanned page after page. I truly used to LEARN stuff! But their quality has gone downhill over the years, IMO. Still a good, solid publication...but nothing like a decade or so ago.
The British ones now remind me of the Macworlds from 8-10 years ago: PACKED with good stuff (even the design, photography, infographics, etc. of the British ones are outstanding...they rarely seem to use Apple's stock PR shots, and always shoot their own amazing photography, showing the stuff in the best possible light).
Too bad they're expensive for us statesiders...
Buy one of each (Macworld and MacAddict) and you'll immediately see the difference in tone/approach. Go with the one that matches your sensibilities more.
Personally, I can't hang with MacAddict AT ALL...I feel like I should be Johnny Knoxville or something, when I read it.