I usually lay it out like this.....
3 piece : (510) if you don't mind the extra attention it requires to maintain the atomizer and the cartridges, then it's great, more vapor than a KR808D-1, and a bit more flavor hit.
2 piece : (KR808D-1) If you're always out and about, or have no time or desire to mess with anything then this is perfect, by far the simplest e-cig available. It costs more to just vape prefilled cartomizers, but you can refill them too or buy blanks to use your own favourite juices. It cost's anywhere from $7.50 to $10 for a pack of 5 prefilled ones, one lasts about the equivalent of 3/4 of a pack of cigs for me.
So really if you want to save as much money as possible then a 510 is for you, which comes along with the extra fiddling.
If you want the simplest option available (KR8) then you pay a bit more for the convenience by using cartomizers.
Both kits can be found for around 35 bux.