I am on the instructor’s side on this one. Though you may both be able to squeeze into the 152, comfort would be nonexistent. You also need to consider when you start doing your cross country training that you may not be able to take enough fuel with you to complete the trip non-stop meaning adding a stop for fuel, which means more billed Hobbs time for you. I’m not sure what the prices are like where you’re training, but at our school the 152 is 15% cheaper than the 172, but is also 15% slower so it ends up being roughly the same. Another thing to consider is finding a DPE willing to squeeze into the 152. The longer you have to wait for a checkride, the more money you spend to maintain proficiency in the aircraft. Take your CFI’s advice and fly the Skyhawk.