From an entirely theoretical point, 'sucking' creates a more even air flow, as atmospheric pressures causes more even air flow, than a fan blowing air through something.
This is why nearly all main vehicle cooling fans are behind the radiator, and it's not just a carry over from the days with engine driven fans where it was the most practical position.

However, in the realm of electronics and enclosures, people normally just throw enough airflow at cooling, as you're not likely to be chasing marginal gains.

Personally, I'd be happy as long as any cooling fan is assisting natural heat flow and not fighting against it, unless there was a specific reason for doing so.