Heyas Tchara,
Basically the biggest difference is the threading of what you can attach to the batts. I know that isn't much of an answer, but there are different voltages available for both threadings.
mods available with either threading. Adapters to let you use an excessory for one on a batt with the other threading, etc. I think most kr8 starter kits come with slim batts. Mind you I said most
The 510 fat batts seem to be so popular around here that you'll probably find more starter kits with 510 threading in either slim or fat batt models. Like the joye 510 vs joye ego. joye 510 is slim batt. joye ego is fat batt. Although there are 808 threaded kits with fat batts, I think. 901 thread?
btw, you can find 808 batt to 510 atty(or carto) adapters at
Welcome to MadVapes LLC for a couple bones each. Like
901/808D Battery to 510 Atomizer adapter
Hope this helps,
Maximillian the Mostly Mischievious