Here is my 2 cents. As far as aggression and personality goes GT and Os are pretty similar. Both are pretty personable, and generally mellow fish with little aggression.
As far as growth rate goes Os win hands down, not only do Oscars grow faster but they also grow much larger than GTs do. GTs are pretty slow growers but if you have the patience it is pretty awesome to have a GT that is 6-10".
As already stated GT are better looking and more colorful, but Os can be pretty damn stunning as well just look at Knifegill's.
The one pro that GTs have over Oscars is that they are not as messy, hence they are lower maintenance.
When it comes to which fish is better it all boils down to personal preference. My vote goes to the GT.
By the way if you have a GT that is not eating, something is wrong with it. GTs are great eaters, they are generally not very aggressive however then can become very aggressive during meal time. The one that I have is very competitive when it comes to food. If yours won't eat it might be sick.