Which AC Hard Start kit capacitor I should use?
Should you just change a cap because your system is old? NO you don’t state there are any problems.
Electrolytic capacitors do fail over time but we don’t know what you have.  oil filled usually last the life of the unit but are much more expensive and not appropriate for all climates.
If you are noticing the lights dimming , hard starts blowing fuses if all the connections are good replace the cap with the same value.
As electrolytic caps age they loose their ability to hold a charge and or short out (note a bulged cap case has usually shorted, it may be working now but needs to be replaced).
As caps loose their ability to hold a charge they can not get the motor started thus dimming lights and blown fuses, let this go in long and the compressor windings will be damaged.
There are lots of film flam guys out there if your system is starting hard the cap just needs replacing with the same size it is that simple  (if the cap is the problem).
If you could not find the same size +-10% will work but not as well as the manufacturers recommended size.