Should I buy an unlocked Iphone or should i just get a different phone?

Well my phone was stolen and there's no way that i can get it back so i went to my phone company which is t mobile just to get a new phone since they were selling iphones i was going to get the iphone 5 but they told me that i would have to get a separate plan from my family plan. I don't want to leave my family plan. So I was trying to find out if it would be a good idea to get an unlocked iphone 5 and use my tmobile sim card but I wasn't sure if as soon as I put my sim card in that they will find out and take me out of my family plan or is it better for me just to get a different phone.

Go on eBay and look for used tmobile iPhones or unlocked iPhones. You can find some cheap or get an unlocked 4 or 4s that's been used, but looks and works perfect, really cheap

I will suggest you to go with unlock iphone. Usually iphone i will go with as its camera and its fast.
AND getting different would also be an idea but as far iphone touch is awesome