The big difference is going to be drainage. If there will be water running alongside the slab there is a chance that it will erode and eventually run under the slab. Building up the pad around it will help avoid that situation. Depending on your drainage you might not have a problem without one though.
One person mentioned a nice thing about not having a pad was putting a door on the part of the barn that is 30" high and using that as a natural loading dock.
I built my own pad site up this weekend for my 30x40 barn. It ended up costing $800 and about 20 hours of work. ($500 of fill dirt, $100 rental of a jumping jack and $200 labor for the kid to run the jumping jack) I had received a quote to do it for $1,200. In retrospect I should have just hired it done, but at least I got some experience using the tractor and felt like I accomplished something. I know I packed it in good and solid with 4 separate layers with packing each layer down good.
I only built my pad about 1 foot extra and then it slopes down at about a 20 degree angle for another 8 feet or so at the tallest spots. This takes care of any erosion problems but it doesn't leave a very nice work area around the building if you need to work on it.
That's my 2 cents worth.
Nathan
