Stay away from that BenQ. Its a TN panel. The Dell is an AMVA and is better. Response time of 2 ms is G2G for the BenQ and that is inherent with TNs but picture quality is not good on a TN panel. And this response time of 2ms 4ms 6ms are something that is under perfect test conditions and involves a lot of parameter which you will never implement. So dont read too much in to it,though of course its important but not a deal breaker.
You are safe with the DELL's modern and superior AMVA panel.