That depends. 510 is the
thread style. You can get 306's and 2.0 ohm cartos in a 510 style. These things you are comparing are all different animals and thus like comparing apples and oranges and cherries, they are all 3 different things.. 2.0 is the ohm rating and these work well at lower voltages like a Ego 3.7 volt battery or a Riva 3.7 volt. Cartos and 306 atty's can come in the 2.0 ohm as well as lots of other ohms depending on what voltage you want to use with it as well as many different
thread sizes. 306 atties can come in a 510
thread size. ( don't know if they come in other thread sizes or not) here 306 is not a rating of threads as I understand it but the style of atty. 306 atty's have a shorter body with a higher exposed coil. ( made for dripping) Someone correct me if I'm wrong.
With this in mind you may want to rephrase your question.