I am going to assume based on this question that your implementation is likely replicating the same values to both a prop and an evar? (this is a really old methodology that most implementation guides recommend against now...)
That said, if this is how your site is set up, and if the eVar is set to a Hit level expiry, there should be no difference (with the exception that props have a 100 character limit, and eVars have a 255 character limit - or maybe your prop is configured as a list, etc).
There really is no preference to which one you use in your segments, that really comes down to the differences that you have configured that should drive that decision.... if the length is larger (like you are storing URL data) I would use the eVar so there is more to match, if you have a visit level expiry on your eVar then you would need to evaluate if you want to match the value for specifically when the value is set or at the visit level, etc.
In some case, you may want to do an OR statement check both values. It really depends on the data, the configuration and what you are trying to achieve.
I don't replicate values, with the exception of transitioning tracking from a prop to an eVar as the value grows. I create segments looking at both, and for those transitioned I look at both the eVar and the prop so that if I am looking back at older date ranges, my rules will work with the old and the new.
