My daughter is 18.5 now but we have all carried IPhones since they came out.
Our plan is bundled together through our company. I have the 4, hubby has 4s, and daughter has the 5. I do pay about $220 a month for everything which included unlimited texting and data and ours is a little pricier because we have international calling for our company.
Since daughter is a paid employee in our company, we pay her portion of the phone bill.
IF you get the IPhone, I strongly suggest the insurance plan which covers you for the accidental drops in the toilet, drops on concrete, etc. Believe me... it happens to the best of us. Hubby nor I have the insurance plan and he has already paid $100 3 times over 2 yrs to replace a cracked screen due to dropping the phone. He refuses to use the case.
We know a local guy who does repairs and he has come in handy often. Daughter's phone is covered and she has walked out with a new phone due to some defect, walked out with a new phone at $199 vs the list price when her phone was stolen.
We love our phones and wouldn't carry anything else. I can run my company on my phone and that comes in very handy if I am out and about and do not have my laptop with me. I can seal a deal and never miss a beat. Keep in mind, I have complete access to the internet and if you go this route, your children will too.. I have never blocked things from daughter, we use communication and so far, she has never wrongly used the internet, etc. However, we discussed the internet and potential dangers so that she is aware.
I think you are looking for feedback and not necessarily answers as to would you yes or would you no allow a teen to have a phone and how much data/text? It is a personal decision for you and your family. What has worked well for us may not work so well with another family.
You are on the right track with gathering all the info you can. With a lot of info, you can make a better informed decision. Best of luck!