Whats the difference 501 and 502?

[The Chef]

Hi there I'm looking at possibly jumping ship if I can't get my SX3 to behave it's self at TORCH's next all day meet. I would like to know the differences please I've seen a 501 worlds and also the 502, I know both of the 50's are shaft and the 11 is as I'm looking at either of the above the 511 is another option but am not sure. I'm also considering the Durango but not so sure.

Any information would be great.

Next thing is,

1, are these cars reliable?
2, are they robust?

Thank you.