MTB levers VS BMX levers... which ones?

Hey guys, I am replacing both brake levers on a friends bike. I found some inexpensive levers on ebay. The BMX levers are much cheaper, which is preferred. I have read that BMX levers differ from mountain levers. Some one called it "cable head". What is "cable head" and how can I tell the difference (BMX vs. Mountain)? Can I measure the existing cable end? What dimension is important? Does it even matter?
Thanks in advance.