Smartphone won't do you much good without a mobile provider? And not all smartphone models are available from all mobile providers.
Notwithstanding, you can use most any Sprint smartphone on RingPlus.net. Most Verizon smartphones on PagePlus, except for prepaid Verizon phones. Can only be a 3G smartphone for pay as you go plan, or 3G/4G LTE for monthly plans.
I'm not sure what a semi smartphone is? I would think an iPhone would be a little easier to use for a novice. And you can get answers to questions on how to use it at an Apple store.
You don't need to buy the most current smartphone. An iPhone 4s or iPhone 5/5s would be significantly less expensive and work fine with both services. Especially, if you buy it pre-owned or off-lease.