All other things being equal, a 256 bit card is better than a 128 bit card, it has more bandwidth for the memory (the "pipe" size than data flows on).
As far as comparing 2 video cards, look here http://www.videocardbenchmark.net/gpu_list.php, put in the models you want to find, you'll get a speed test result and a rank based on the other cards in the chart.