DCT growing now... Probably before ICO will grow more. Better to buy DCT now 
This is who already has DCT, the easier it is. And so, you will lose more on fee. It is necessary to send BTC for exchange, buy DCT. Even with Bittrex is not so easy to withdraw. Although DCT is on HitBTC ... But how to buy tokens for DCT from wallet exchange? Because of 10% of the bonus you can get into an uncertain situation. It is better to buy for ETH.
What is the rationalle behind offering bonus when purchasing with DCT?
Both companies belong together so you get a bonus by buying in with their own token I assume.
What do you mean by belong together?