Yep the only difference is speed. No difference between cables.
Only SATA 6GB/s devices will make use of 6GB/s connections though. For example, if you plug a SATA 3GB/s HDD or SSD into a 6GB/s port, it will still only run at 3GB/s as the device is limited. Same for if you plug at SATA 6GB/s HDD or SSD into a 3GB/s port, it will only run at 3GB/s, because the connection is limited.
And there won't be a difference plugging a SATA 3 hard drive into a SATA 3 port, or into SATA 2. The only difference you'll see is when you get a SATA 3 solid state.
I really appreciate all the responses.. Learned a lot!