I wrote a site that automatically made thumb nails of images that users uploaded. I chose to use imagemagick so I wouldn't have to deal with any additional php requirements learn any complicated gd functions so I could just execute a simple shell command and be done. Also, I knew image uploading would be rare on this site anyway as it was for a bunch of my friends to share pictures. Maybe 1 image a day, if that.
If you really worried about speed, the internal gd functions are the way to go, but will take more time to learn and implement most likely.