In his last post comparing GD and imagemagick, Christian mentions some of the flaws that imagemagick had over GD. Well, in this new post he takes back some of those opinions, stating that he didn't realize a few things.

After further investigation into imagemagick (here's the last post to this matter), I realized, that you can improve performance quote a bit, if you set the correct compression quality. With a quality setting of 30, it was equally fast as GD *and* the picture was a lot smaller.

What do we learn from this? Imagemagick (and the php extensions) are very powerfull and the correct settings are very important. If size and/or speed is important to you, it's a good thing to play with those settings and benchmark them with your pictures. GD doesn't have a setting for the compression of PNG images, btw, so you always end up with quite large PNGs compared to imagemagick with correct settings or pngcrush.

So, it's a case of different tools for different jobs, not necessarily a comparison of the two. Can they be used for the same thing? Sure - but you're not going to get the same results when they each seem to specialize on one side or another...