Good thing you are asking this question on a United Parcel Service UPS site instead of a United States Postal Service USPS site because who knows what kind of inaccurate information those postal employees might give you.
One you just cover vacations primarily for people who provide their own vehicle on rural routes which you would have to provide your own. The other you're a part time fill in for rural routes. You get no hours basically for both. You're better off going for a City Carrier assistant position.