I was going to write an answer saying if speed for transfer was important for the large backup files then use SATA-3, but after looking at this paper http://www.seagate.com/docs/pdf/evolution_to_sata_6gb_storage.pdf I'm not sure if the interface speed is going to make a difference, because it is the hard drive that is the limit.
Why Is SATA 6Gb/s Needed Now?
Today, hard drives for PCs don’t seem to be pushing the limits of the SATA 3Gb/s interface. The highest-performance desktop drives on the market deliver about 1.5Gb/s in sustained data transfer rates. Add another 0.5Gb/s in command overhead that is not available for general data transfers, and you still have 1Gb/s of headroom before you bump into the 3Gb/s ceiling. But it won’t be long before hard drive technology catches up to the SATA 2.0 standard. Seagate estimates that hard drive transfer rates will exceed 2.5Gb/s by mid-2011. Add in the 0.5Gb/s command overhead, and the SATA 2.0 standard is out of gas.
The PDF was dated January 2010.
The did do a comparison between SATA-2 & SATA-3, and 32MB & 64MB, but unfortunately, they compared slow interface and less cache to faster interface and larger cache. It would have been interesting if they did the comparison you were looking for.
Unfortunately, I think looking at the hardware websites and getting independent numbers on the actual throughput is the only true way to know.