Hi. Using both. Graphicsmagick is slightly better in terms of efficiency, but I think they basically stuck at the feature set from the time when they forked from Imagemagick. That said, I'm missing some functionality in graphicsmagick (especially the "aspect correct resize to fill given area" for thumbnails).
Unless you run a really high load server or have special needs for new Imagemagick features, you won't probably recognize the difference (given you install the Imagemagick compatibility "layer" and use it in "eZPublish" way).
