1987 Cherokee= 4 cyl. vs. 6 cyl.???

Considering buying a 1987 Cherokee 4 cylinder with 147K miles for delivering mail for the post office. Does anybody have any experience comparing the 4 and 6 cylinder Jeeps for durability? I know the 6 cylinders are known as iron horses, I have one with 178k and one with 168k and they both run great but the gas prices are a killer.
Thanks for your time and any help,
Zack

Don't expect any power out of the 2.5L 4 cylinder. MPG might not be as good as you'd think either, I got 17mpg in my 97 4cyl Wrangler, highway or city, and got better mileage with our 4.0, up to 23 highway.