A battery cap gives you power surges making your amp have more output and save a lil drain on your battery. I would say the diffrence is wiring and the need? Anyone else wanna talk bout that?
best electrical upgrade is a new alt. and a deep cycle battery. if you really need more backup power after that then look into the batcaps. i plan on running 1 batcap 400 series to each of my 3 amps when i get my box built for my DD. only reason i am going with 3 is b/c i can get them for 125 a piece and thats a pretty good price so i figured why not make sure i have enough juice. i also believe loyd stated before they are mainly for systems with large amounts of wattage going through them??