The part that still confuses me about your situation is why you aren;t comparing TMO's smaller data plan with Sprint's 6GB. Their 6GB plan is $65. Is that not eligible for the $20 off that your friend is able to give you? If it is, seems to me, that is cheaper.
You seem to just want the cheapest plan and what we've tried to do here is break down what can possibly be the cheapest. You haven;t yet mentioned why you went with unlimited on tmobile but could live with 6GB on sprint. I imagine you could live with the same 6GB on tmobile, unless I am missing something.
The 6GB plan on T-Mobile is only $15 cheaper than what I am paying now. Sprint's total bill would be $32.50 a month for 6GB of data, plus my EIP.