Average case performance vs. Worst-case guarantee

Hi, I have this question to which I have not been able to get a conclusive answer (I have heard different things). When an ISP buys a router does it want a worst-case guarantee about the router's capabilities? Or will it buy a router which can give better performance in the average case (it may drop some packets if the traffic pattern changes suddenly)? Assuming both cost the same. Could you please give your opinion? Harsha.