Not too much, the suspension was retuned for 2007 and then again retuned for 08 and 09. 2007 was 1st year for navi and heated seat option. Rim options. Some other minor things here and there nothing that worth looking at one or the other if you are able to get a really good deal on one.
07+ models have more trim levels, Shelby, GT500, Bullitt.
07+ models have a different computer which requires a different tuner (if you go with a diablosport) and in some supercharger packages an install kit for them.
07+ models have also more chassis stiffening then the 05-06 models.
07+ models have a better and simpler designed routing for the cooling system.
07+ models come with a TPMS system.
07+ models are usually more expensive.
latter 08-09 models should have the redesigned cylinder head with the more traditional one piece spark plugs which don't fail when you try to remove them.