At first its all about finding the batt style, you like best. I started with the ego, but found the analog feel was important, so went with the kr808d-1. It doesnt need as low ohm on the carto as the ego does. Gives a better hit. Once you got the batt down, its all about experimenting with performance of attys and cartos.
Vaporsetc has a plethera of different cartomizers to choose from. Both high and low ohm and diff manufacturers. High ohm gets the flavour through better and low gives a stronger hit.
You can get both 510(ego riva etc) threads and kr8 with cartos, so no matter witch you choose, you have alot of options. Since its all down to personal performance its onlu by trying all, you find out whats for you.
Egos and rivas, require you can leave the analog feel behind you. I couldnt do that, but they are some very dependable units.