It doesn't really matter, What you're referring to is push / pull. If you google it there has been a lot of ongoing discussion going around in circles!
I personally prefer pull, it's easier to clean the rads although I'd argue only slightly. Currently my rads are all in push as I had to go that route to get everything in my case, a blast of air still gets the dust out.
Do what you prefer!
Push + Pull (a bit of both) is the best solution, but it's most useful on thicker and/or denser finned radiators. It allows you to run the fans slower and keep noise levels down. The differences are still relatively small though.