ImageMagick vs GD - which is faster, less resource intensive and produces better images?

I would lean towards ImageMagick as far as image quality goes. It seems to produce considerably sharper/higher-quality images than GD2 (at the expense of larger filesize.)

ImageMagick is also not bound by PHP's memory limit.

If in doubt, use ImageMagick. I haven't observed any big differences in speed and resource usage, but IM is way more flexible, better in quality in many areas (although you can expect decent results in watermarking from both libraries), and supports way more formats.

Many advanced operations that are complex or impossible to achieve with GD are easy to do with ImageMagick.

On the other hand, ImageMagick is not supported as well as GD by many hosting providers, so you'd be adding a sizeable dependency. Depending on the nature of your project, that may or may not be a problem.

I have built GD and ImageMagick libraries. Both work just fine and produce the same size images if used right. However, ImageMagic comes with a lot more options out of the box and is what I lean towards now.

In addition, ImageMagick works great as a stand-alone for bash scripting or another terminal functions which means what you learn is useful outside of PHP.