Bro
As a rule of thumb we add capacitor of 1 farad for every 1000wrms of system power .. Is good to add a capacitor to system , reduces the load on car electrical system in SPL burst ,and insures the amp power it needs , less voltage fluctuations , and gradually increases loudness (in my case as I've experienced it)
Bro it's good to invest in high quality brands like the above you have mentioned , reason being other brands like boss ,power aucostic , spl precision power etc are likely to over rate their capacitors ..the actual power output maybe less ..
Also nowadays hybrid capacitors are in the market look into them , they perform better then the previous stiffening digital capacitors ,charge and discharge better .for you anything is good between 2-10 farad . 5 or max 10 farad hybrid cap is way to go , more than enough for both of the amps ,
Stinger SPC505 5 Farad Pro Digital Hybrid Capacitor
Tsunami HCAP-5 Digital 5 Farad Cap Car Audio Capacitor
Rockford Fosgate Caps and Power Products: stiffening(tm) capacitors, power inverters. Car Audio for Fanatics. - RFC10HB - Rockford Fosgate?