I just got an iphone4 last year before traveling abroad for the summer. my advice: its much simpler to just get a cheap $5 phone there with a pay-as-you-go plan and use your smartphone when back in the US. I still kept a plan where I could receive calls while abroad so wouldn't miss any important calls. Also, there is a free app called Textfree where you can send texts via wifi instead of paying for the network. Also, if you have wifi you can use all the cool apps including skype. While the iPhone4S is the newest, in real life the iPhone4 does everything you need and will most likely drop in price soon
for carrier I would say stay away from AT&T, i've had dropped call issues with them that I never had with Verizon
edit: P.S. whatever smartphone you get, I've heard from students and physicians that epocrates and medscape are the most useful medical apps to have. some hospitals even have iphone apps available to doctors to allow them to check off which patients they have seen throughout the day
for carrier I would say stay away from AT&T, i've had dropped call issues with them that I never had with Verizon
edit: P.S. whatever smartphone you get, I've heard from students and physicians that epocrates and medscape are the most useful medical apps to have. some hospitals even have iphone apps available to doctors to allow them to check off which patients they have seen throughout the day