[2.5" HDD] vs [3.5" HDD + SDD] is an odd comparison.
500GB 5400rpm 2.5"
1000Gb 5400rpm 3.5" + 16GB SDD.
Really, why the SDD for the latter option and not the first? 3.5" HDDs (even the 5400rpm models) are faster than laptop drives (or possibly on-par with 7200rpm laptop drives but those aren't available in high capacities, and they cost quite a bit more, and have the potential to make more noise than 5400rpm 3.5", thus in every sense worthless for desktop) so you'd benefit more performance boost with a laptop drive.
Use of SSD alongside a HDD locally is never a silencing choice (as one HDD without SSD would be equally quiet) but always a performance choice. If you would be fine with single 2.5" 5400rpm... then trust me, you will be fine with single 3.5" 5400rpm. No need for SSD.
Like Eunos said, advantage of 3.5" + SSD is speed and capacity
. Not quietness. Thus, the option of single 2.5" is really not a proper participant for comparison, as the only reason to install 2.5" HDD to desktop is for quietness.
Of course 3.5" + SSD can be even quieter if the HDD is connected via network and the server is buried deep in a closet somewhere far away. Then there's really no sacrifice in either performance, capacity or noise. Only more money, extra power consumption, space needed to store the server, etc. Local (non-networked) 3.5" + SSD is really only for gamers, not silencers.
To determine which of the two options is better, we have to now more about the criteria for being better. Is money an issue? Won't networked mass-storage be a bit of a nuisance? How low is your noise tolerance?
You said you have 7200.10. Well, comparing to any 5400rpm 3.5" HDD, 7200.10 will be like a friggin' jet engine. Going down to 5400rpm 2.5" it will become even quieter. As for people on SPCR not using 2.5" HDDs that often... I think that's mainly because the 5400rpm 3.5" did a sudden, completely unexpected comeback all of the sudden, and this time, for the first time in history, they were coupled with the quiet bearings 7200rpm drives have had for years. So, while 5400rpm 2.5" is still quieter plus easier to silence even further, 3.5" is more convenient for hard-mounting, cheaper per capacity, bigger available capacities, better performance, etc. Back when it was either 7200rpm 3.5" or 5400rpm 2.5", the laptop HDDs were a proper choice (even though the performance loss was more noticeable than it's with 5400rpm 3.5" vs 5400rpm 2.5").
I have built one computer that runs on 120GB Samsung laptop HDD. Occasionally I've used 2.5" Toshiba on another of my computers (though at the moment, it's running 3.5" (P80) again). Even low-rpm 2.5" are pretty decent. Especially if it's for home-theater, office, light gaming, etc. Only for stuff like video-editing I'd take stuff like performance into consideration when planning mass storage solution.
And on the other end of the scale, how many regular users need anything near 1TB of storage today?
Lol. Here's 7TB and I'll probably have to buy more soon.