The ShopVac seems to have a little better suction over the Rigid but that's probably because it has a smaller diameter hose. It's good for cleaning out a car, cleaning carpet steps, cleaning furniture (with the right attachment) and definitely good for cleaning up wet spills (sponge type filters that fit the ShopVac are fairly inexpensive and I use one underneath a cloth type filter bag).
The Rigid doesn't have wet sponge type filters which are inexpensively available (the filter on it is more proprietary) and it also doesn't come with a wet filter so I don't use it for wet spills. However, where the Rigid shines is when you have to pick up dry debris which would quickly clog the small hose on a ShopVac. It is a beast on stuff like styrofoam popcorn, gravel, glass and rocks! Anything that is too big to pick up without clogging the ShopVac hose.
Hope this helps..........