People say that ImageMagick is much faster. At best just compare both libraries and measure that.
Prepare 1000 typical images.
Write two scripts -- one for GD, one for ImageMagick.
Run both of them a few times.
Compare results (total execution time, CPU and I/O usage, result image quality).
Something which the best everyone else, could not be the best for you.
Also, in my opinion, ImageMagick has much better API interface