>>857745 (OP)
>-small inside PC, also allows you to have smaller case
Irrelevant, once you account for airflow and wiring
>-small and light when you need to transport it, can easily be put into jacket pocket
Valid, if you need to haul TBs of data. Even then it might be simpler to run "ssh 'tar c' | tar x" at night than bother with copying/moving/screwing HDD in. Or good-old USB-stick
>-quiet and low vibration
Do not depend on form factor, but on RPM
>-low power
Ditto
>-can easily be hidden in your house when police knocks on your door (because it's small and thin)
Irrelevant, not small enough for that purpose
>-can be put into laptop and PC, or used as external drive
Irrelevant. Do you plan to screw HDD out of PC every time you use laptop?
One pro.
>-big, can store a lot of terabytes
Valid
>-heavier, less prone against damage
Irrelevant, weight does not give shock resistance.
>-harder to be stolen
Irrelevant. Lock your fucking door
>-cheaper per terabyte
Valid
>-can store a lot more on single platter
Repetition of (1) and (4)
So 1 for 2.5, 2 for 3.5.
3.5 is the solution.