unlocked Japanese smartphone
 

My son is going to the US for a year, and he's thinking about getting an unlocked smartphone in Japan before leaving. He says he can get one for 10,000 to 30,000 yen. Not an iPhone, but something else. Then he'd get a SIM card in the US. I'm wondering if it is better to just rent short term a smartphone once in the US. From AT&T or something. Any thoughts? Thanks in advance.