<p>I've build a 3D printer from sourced parts and mounted the hotend cooler to blow air over the heatsink. </p>

<p>Talking to a friend, he said it's better to reverse the airflow over the heatsink, but couldn't give me an argument other than everywhere he saw it was like this: all coolers are mount to suck the hot air away from the heatsink.</p>

<p>Is it one way better than the other way ? And if so, why ?</p>

In the case of 3D Printing, it's going to be faster to cool the radiator by blowing out the heat from the source.
The idea is that you're trying to get rid of as much heat as possible in the quickest means possible. By blowing away from the radiator, you're allowing the ambient temperature to cool the hot air being blown out.
If you were to blow the ambient air towards the radiator, the blown air will warm slightly as it is overwhelmed by the heat of the radiator. Even though the ambient air may be cooler, it will take more time to cool off the radiator.
Example of bad airflow, which will take much longer to cool the radiator:
Example of good airflow, allowing the hot air to be quickly cooled by the ambient temperature of the build space: