I've been using ST for 3 yrs now, using Verizon towers and couldn't be happier with the service. Want to upgrade to a Smartphone. Love the IPhone 5C but not sure it would be worth the cost and want to be sure I can stay with Verizon. Also see great reviews for the Samsung Proclaim. Lower end but looks good.
Although your iPhone on Straight Talk may contain a SIM slot, DO NOT insert a SIM card into it. Your iPhone on Straight Talk works on a CDMA network(Verizon). From their website http/iphone.straighttalk.com/