I have an Iphone 5 that I bought from Straight Talk. I also have the Straight Talk monthly plan. It runs on the Verizon system. I specifically wanted to be on Verizon so I picked a phone that runs on their system.
As has been stated -
1. The phone you buy from STRAIGHT TALK drives which carrier you end up with. There is no choice. Cost is $45/month.
2. The rules state you cannot use the phone as a hotspot. I use Millenicom.com for data which gives me the portable MiFi. I have the phone set to search for it's signal and use it if available.