For T-Mobile USA, you can obtain only a micro-Sim card/activation kit for your iPhone 5 (see previous post above) - the $3 a day plan unlimited will give you 4G speed for only the first 200MB, then slowed down to 2G speed (that will be slow - as in H/E or GPRS) !! or $42 for 14 days. $50 for 30 days - or $60 for 30 days with first 2GB at 4G speed and add'l unlimited web at 2G. If you can estimate your data needs or usage, you can figure out whether it's going to work for you or not as 200MB will go quick - but if you will have access to WiFi indoors (hotel & office for extended usage) - it might work. Some folks stream non-stop and will burn 1GB data very quickly - all unlimited prepaid plans with data will throttle the advertised 4G LTE speed down to 3G or 2G after a preset limit (in the fineprints) is exceeded.

AT&T's $50/mo. plan is for non-smartphone only, it's $65/mo. for prepaid non-smartphone with regular or micro Sim card and up to 1GB data. There's a $50 no-contract tablet & data only plan but they might not allow that on the iPhone 5 or prompt you to spend more.

For both T-Mobile & AT&T prepaid options for short-term visitors, it's advisable to bring an unlocked, inexpensive quadband GSM phone (regular Sim card, to be carefully cut/trim for micro-Sim devices) Their networks can identify & sweep your MEID/ESN and block/prompt you to switch to a proper $$ plan for data, especially if high usage raised a red flag ...

http://prepaid-phones.t-mobile.com/p...ll-phone-plans

Also, please check your device's 4G/LTE and bands to see whether it's compatible with USA bands/frequencies, as I know most/all Asian carriers are on the 2100Mhz unlike USA carriers, so you might only get 3G or 2G speed.

Generally speaking, once you obtain the correct micro/Sim card and get it activated with a starting balance, you can usually switch back & forth among daily or pay-as-you-go plan to start out with, and pick the monthly ones once you are satisfied with how it's working.