Thanks for all your replies.
I already had a spare computer, so I ended up buying a Radeon RX480 to mine altcoins and exchange them in Bitcoins.
A RX480 (~200) with the rest of the computer consumes 240 watts and can "mine" 0.008 to 0.009 BTC per day.
If I went for an Antminer S7 (~350) and underclocked it, I could of mined 0.00072117 BTC per day (1.6Th/s @ 400 watts).
I ended up spending less, using less power and getting more BTC per day.
i single readon can't mine 0.009 per day, that is insane i think you confused the amount, the return is 0.0015 per day
this is always better than 0.006 from an antminer that consume 1400watt, that is 4x one antminer s9 while consuming almost 1/10
i think it's better to wait for bitmain to release the new antminer s10 or s11 with better efficiency, i hope they make more model for lower consumption too