I would say plenum core Cat 6 because its a pain to work with and you really gain appreciation for the people who can work with those cables XD
You shouldn't bury a regular cat5 cable. There is some cat5 made specifically for burying. You could also put it in a conduit.
If you're going through the trouble of burying a cable, definitely make sure you run the best cable you can afford. A conduit is a must, also. This way you can always pull at least one more if need be. And don't run stranded cable either. And no 90 degree bends!