The 510 is smaller than a kr8. The pics make it look the other way around, but it is a lot smaller. Either one will be good for you, but for different reasons.
510: more vapor, easier to add juice to carts.
kr8: perhaps more flavor (depends on several things), easier to use (2-piece) on the go.
Both ar great PVs. I'm a 510 guy myself. Love it.
Its always best to check here about such details. If you don't have any experience with PVs, the pictures can be misleading. I thought the 510 would be large too. Its about the size of a 100 analog.
I ended up getting a KR8 from V4L for hubby and I ordered a 510 from Cignot for myself....he is gonna be quite upset if thats the case lol but I figured it would give us both a chance to try out each one and see which one worked for us