Depends entirely on what speeds you normally Rev the fan to, and what kind of fan. If your normal speeds are 1500+ then push is more effective, if your normal speeds are 1500- then pull is more effective. Also take the fan into consideration. If you use a directed flow style fan like a Noctua nf-f12, then push is always better, if you use the stock rad fans then it's the speed that counts. Either way, I'd use the fans as exhaust not intake. Absolutely no point in putting warm air back into a case.