Shapeshift is very convenient but the fees are exorbitant compared to other platforms.
Shapeshift: 1 BTC -> 14.20477011 ETH
Evercoin: 1 BTC -> 14.25967407 ETH
Changelly: 1 BTC -> 14.35434362 ETH
While Changelly appears to have the best rate they don't guarantee the rate. In my experience the actual rate is often much worse than the estimated rate (and unfortunately it's too late to cancel once you see the rate).
Shapeshift: 1 BCC -> 0.06773499 BTC
Evercoin: 1 BCC -> 0.06682701 BTC
1) pick the cheapest exchange 2) sell at expensive exchange price 3) add commission 4) add network fees
Picking a reasonable bid-ask spread is hard, and probably involves more input factors than the 4 you listed above.