Ahead by how much, exactly? Single digit percentages? Is the workload that is targetting the "quietest mechanical HDD" going to be so performance sensitive that a few percent is a terrible issue? There were a number of cases where the 3.5" drives and the 2.5" drives were within 10% of the first generation of SSD drives in that same benchmark. Also in that benchmark were other 3.5" drives of varying age and capacity, and the 2.5" spindle drives were having no issues keeping within single-digit performance differences.
I firmly believe that you're trying to split a very fine hair here. The first post asked about the quietest of drives, which stemmed worry about heat in an enclosure, which equates to power and spin speed. Given all the criteria mentioned, why are we even having this discussion about "OMG a 2.5" drive is a few percent slower than a 3.5" drive?"
The 2.5" drive will be quieter, will use less power and generate less heat while maintaining the performance that is expected, hands down.
Obviously speed isn't a factor for the OP. His requirements is a quiet drive to use for backups and data storage.
In which case the best value for money is a 3.5" 5400 RPM drive in the 2-3 TB range. If money is no object and you don't need more than 1 TB of storage then a far more expensive per GB 2.5" drive would be suitable obviously.
So, yes, I agree, as long as his storage needs fits within the storage limitations of a 2.5" drive then it could certainly be the best choice.
The whole sidetrack of speed was brought up by you. And no, any modern 2.5" 7200 RPM drive would not match up well against any modern 3.5" drive. To claim otherwise is patently false. Well, unless you compare only the slowest 3.5" drives to the fastest 2.5" drives. And even then it isn't always close.
Compare the results from this in the HTPC and Gaming capture tests.
A variety of 2.5" 7200 RPM drives, the fastest on the market as of March 2013.
http://www.storagereview.com/seagate_sshd_thin_review_gen3_500gb_st500lm000
A variety of 3.5" 7200 RPM drives, the fastest on the market as of May 2012.
http://www.storagereview.com/hitachi_deskstar_7k4000_review
The results aren't terribly close. The only 7200 RPM drive that compares favorably is the Hybrid Drive with 8 GB of flash and even that doesn't do well in the HTPC tests.
Excluding the hybrid drives. In HTPC the best 7200 RPM 3.5" drive is about 108.4% faster than the best 2.5" drive. In gaming at least it's closer where the best 7200 RPM 3.5" drive is only about 26.4% faster than the best 2.5" drive.
Comparing the slowest 3.5" to the fastest 2.5" at least the gap closes to only a 12.7% performance advantage. The Hitachi 5k4000 is a 5400 RPM drive. There is still a quite sizeable lead in the HTPC trace at about 48.7%.
And this is with 3.5" drives that are approximately 1 year older than the 2.5" drives.
And looking at that it's definitely easy to see why Seagate are abandoning the 7200 RPM 2.5" drive market and instead focusing their efforts on Hybrid 2.5" drives.
Regards,
SB