Since there are many opinions, i will break this down for you,Poloniex , bittrex ,kraken etc are exchanges and you have to spend transaction charges sending the coins to and fro while shapeshift and changelly will convert your coins pretty fast without any hurdle and you will get the coins at the exact market rate and you could save money doing so.
So shapeshift or changeably will not charge me transferring btc? I had transferred btc to poloniex and it deducted some amt of btc and it charged me again when I exchanged them for eth.