Bittrex just charges a flat fee, where Coinbase adjusts its fees based on how congested the network is. I think this is the best possible approach, because if the network isn't as congested, the fees Coinbase charges drop significantly.
On another note, services in this case can't seem to do anything right. If they charge low fees, people complain about slow confirming transactions, and if they charge higher fees, people complain about the fees being too high.