The spindle speed(rpm) will help with access times(very good for a boot drive or even program/game drive.).
That said, if you are just storing file on the drive, you do not generally need fast access times.
My main drive WD Black 2TB(picked up before the floods so it was a great price) I also had a 64gig SSD for cache, but not that is for windows 8 and my files sit on a Seagate LP(5900 rpm) drive. I also tossed in a 128gig M4 for some games and other things that need speed.
This has caused me no issues as the LP still has very good throughput and fragmentation does not get bad on a drive that just stores my personal files.
As for SATA6, I have yet to see a HDD pass the SATA3 spec, but they all come with SATA6 now anyway. Seagate's newer drive(the on they say saves power without loosing performance) has sequential read/writes of about 160megabytes/sec.