Did I give you this link comparing them?
http://www.e-cigarette-forum.com/fo...st-electronic-cigarette-first-time-vaper.html
In their standard lengths, the KR8's battery lasts longer (280mah vs 180mah) because more of KR8's length is its battery
KR8 shorty and 510 standard 180mah
KR8 standard and 510 mega (xl) 280mah
KR8 xl 380mah
And there are fat battery version of both with 6-8 hour battery life and option to use LR parts
If you get KR8 you will have to buy stuff more often - cartos (blank or prefilled) - cartos are about 1/5 the price of attys and you may get 1-14 uses out of carto.
If you get the 510 you can try its normal atty+cart combination or get blank cartos for it or a small selection of prefilled cartos here
Xpress 510 Cartomizers
And you can put a 510 to 808 adapter on it if you want to try KR808D-1 cartos
KR8 has a larger selection of prefilled cartos because it is a 2-piece unit - see carto sections on left here
Vapor4Life-Because it's your life - E-Cigarettes/Smoking Alternatives
With an adapter you can put a 510 atty on KR8 battery if you want to try 510 atty and carts
What are you looking for? If you are looking for bigger hit, then consider the fat battery version of 510, the Ego or Riva 510 because that gives you the option of low resistance attys
If you just want to try something different, try KR8 - that was my second