More coils are probably going to have a higher wattage, give you bigger clouds, suck down more juice and battery. Depends what you are looking for and what you are comming from coil wise.
My baby beast came with the T6 and (forget what it was called the 4 coil lower wattage setup) I liked the economical juice and battery use of the 4 coil but the T6 just hits beautifully, large flavorful clouds. Uses a lot more juice and somewhat more battery life. Havent tried the t8, dont think i need anything more powerful than the t6 (its the .2 ohm if theres multi resistance)
Obviously if your craving the biggest clouds you can get go with the 8. Im loving the T6 im using now though.