WD Green is crap, it runs slow @5400rpm etc and i had to replace 2 of them, even though i only used them for storing purposes. People commend WD Red drives a lot, although i haven't purchased and tested one myself. For storing stuff and low noise you can try Red or stick with green.
Seagate one is the fastest here, although 32\64 mb cache doesn't have much difference.http://www.harddrivebenchmark.net/high_end_drives.html
Seagate one scores = 1392 points
Tos. DT01ACA100 = 1247 (2TB one*)
Disk buffer has its uses:http://en.wikipedia.org/wiki/Disk_buffer#Uses
More here will help in AHCI mode which support NCQ, allowing the user to give multiple commands while the drive is under load. But overall the difference is not that much.
64mb wouldn't make it twice as fast in practicality. Although it can hold twice the amount of data but when the read head is in the right position the size of the buffer matters very little as the data will be streamed immediately from the platter to the buffer and then to the I/O bus at the same rate which marginalizes the buffer.