The Galaxy SIII is more of a software upgrade than a hardware upgrade. Many website put the GPU of the SIII as the Mali-400MP, which is the same GPU of the current Galaxy SII. Plus the Amoled HD got many defects, I using the Galaxy Nexus by the way, so think that the problem of the Amoled HD will still appear in the SIII. Some of the screen problems are that at the lowest brightness, the whole screen will have purple tint. My friends G.Nex got those bandings on the screen, sent in to Samsung, the technician say is normal... Overwise the G.nex is a good piece of hardware with many dev support.
And the quad core is not really optimised on the current Android ICS, can search for the comparison between the HTC One X LTE and GSM version and clearly the Snapdragon S4 which is a dual core perform better than the Tegra 3 Quad core.
So unless, you are still using Galaxy S era phone which is a single core CPU, then this may consider to be a good upgrade, if you are using Galaxy SII or G.Nex, not really worth to upgrade