if you dont like to tinker and want something that just does what it needs to do get an iphone, im sure youll enjoy it, it combines phone calling and entertainment.
if you want more flexibility, am novice to power user and enjoy geeking around with technology the S3 will be a delight.
best of all, check out both, ask important questions and establish what needs are specific to you.
is it battery life?
is it screen size?
is it speaker phone volume?
is it storage expandability?
is it processing power?
is it size, finish?
camera quality?
apps?
cross compatibility and connectivity
etc etc
review here:
http://www.trustedreviews.com/opinions/iphone-5-vs-samsung-galaxy-s3
hardware specs compared between the two here:
http://www.itproportal.com/2012/09/12/iphone-5-vs-galaxy-s3-spec-comparison/