Purely for mining, I'd go for the card with the smaller memory. They tend to be a little cheaper, and I suspect that it will save some power consumption. Downclocking the memory also has no effect no hash-rate, but can give substantial heat and power savings.
actually, downclocking reduce memory latency in Radeons case and for mining context thats far more handy, than throughput[which is matter in case of gaming workload/appz]cuz tiny amounts of data need to be moved VERY quick.