I'm familiar with 2 ez pad installations. One pad is over three years old, the other is just past two. Both seem to be standing up ok. The pads if I recall cost around $500 for 8 X 8 size- they came in two sections that are taped together. In one installation, the owner excavated a few inches of dirt and replaced it with tightly packed sand, and put the pad on that. The other one just placed the pad on the ground after removing the sod underneath, and doing a bit of leveling.
Your choice, but for the extra couple bucks, I'd go with concrete.
Brewman