I'm afraid that SSD are too new and too insecure... so I want to test them, but having a parachute
Against unsafety just back up regularly: I'm quite satisfied at desktop level by my Seagate FreeAgent GoFlex USB 3.0
There's nothing more or better you can do. IMHO.
I want to use the fast HDD for installing software that doesn't fit inside just 128gb and the OS folders that, for saving SSD health, doesn't have to be on the SSD like the temp dir
In the last 80 days (with a powe-on state of just less than 35 days, 656GB written and 141 power-cycling), SSDLife says that my SSD "Estimated Life" is reduced by... 80 days (from 8 years, 3 months and 10 days to 8 years and 20 days), while I have temp and swap on the SSD itself. So don't get foolish.
About responsiveness: I have more than some firm doubts that you'll be able to notice any performance difference between a 7200 or sub-6000 data drive, but it's just my opinion/experience. Take also note that you may go RAID, if in case, or short stroke a larger green disk to have a smaller but more performing one (banking on the same quietness).
About storage space: may you take a screenshot of actual occupied space on your current drives?
By the way, if every 7200 (also the samsung) it's too noise I think I'll go for a different solution
If you currently have a standard peecee, a quiet PC with a 7200rpm drive would turn out to be.. a lot quieter.
But then, with a much more lower baseline noise, you won't quickly stand anymore the drive's noise. IME.