Your best bet, in general, is to get a Nexus-branded device. Those are straight from Google, run vanilla Android, and tend to get updates more quickly. Additionally, they are frequently among the most hacker-friendly Android devices, so custom firmwares and the like from the modding community tend to spring up quickly, so you tend to have lots of options about the software your phone runs as it gets older.
Often times, phones from Samsung or HTC or whatnot come with a customized version of Android, which has been made to look visually distinct, has some additional value-add packages, and whatnot. Most power users will tell you that they prefer vanilla Android. That said, most all the popular devices have aftermarket builds based on vanilla Android (AOSP), or a juiced-up fork of vanilla Android (CyanogenMod, my personal preference), and it's not that difficult to replace your factor-installed firmware with one of these packages if desired.
The S3 is a great device, though, and I don't think you'd regret it if you got it, but if you're looking to get an opinion of Android, a Nexus device is the way to go.
Often times, phones from Samsung or HTC or whatnot come with a customized version of Android, which has been made to look visually distinct, has some additional value-add packages, and whatnot. Most power users will tell you that they prefer vanilla Android. That said, most all the popular devices have aftermarket builds based on vanilla Android (AOSP), or a juiced-up fork of vanilla Android (CyanogenMod, my personal preference), and it's not that difficult to replace your factor-installed firmware with one of these packages if desired.
The S3 is a great device, though, and I don't think you'd regret it if you got it, but if you're looking to get an opinion of Android, a Nexus device is the way to go.