What's the difference between "Sample Straight Edges" & "Sample Even Lengths"?

Well speaking truthfully, there's a bunch of settings no one ever uses, they are super technical and 99.99% of the time you don't have to change. I bet those are of that kind. The wiki says this though:

It is about how the Edges are 'subdivided'...you can see the effect in Edit Mode after Applying the Bevel Modifier):

And:

And I must admit, that I didn't know this, before you asked...

Like Omar said, you'll probably never touch those Settings.

And be aware, that Sample Even Lengths can alter your Custom Profile.