Can anyone explain the difference between an eVar and props?
eVars persist, whereas s.Props do not. s.Props can use pathing, whereas eVars cannot.
For example, if you figure out a visitor's demographics, such as age or zip code, you'd want to put that in an eVar, so that when they convert, you'd be able to see this data.
If you're measuring traffic, like link clicks, actions, etc. it would make sense to use an s.Prop.
Sometimes it might make sense to use both for the same data.
Reference: Comparing Props and eVars
This is great, thanks nikitarama
I do want to point out that eVars can be pathed in Analysis Workspace, just like props.
One advantage that props do have, is the ability to enable them as list props - which can capture multiple dimensional items in a single hit: List Props
eVars have the ability to have Allocation, Expiration, Merchandising, and Counter settings applied. More details here: Conversion Variables (eVar)
I'm seeing less and less the need to copy values to *both* an eVar and a prop. Most likely, you'll want to use an eVar if there's a need for persistence, if there isn't, a prop will do.