I'm pretty happy with my SIII. As long as Samsung (and Sprint) keeps supporting the SIII in a timely fashion then it will give me confidence to purchase future phones from them.
The SIII has been a major success for them so I see no reason for them to stop supporting it, at least well into 2013.
But saying that, if Google eventually releases a Nexus 4 on Sprint with LTE and bump up in memory I may have to take a long look at that. I picked up a 32gb Nexus 7 tablet about 2 months ago and have been pretty impressed with the stock Android experience.