The length of the tables is the main difference and the longer the tables, the longer the material you can accurately joint. Mass is another difference and the heavier floor models will vibrate less and give cleaner, smoother cuts. Power may be a third difference, since all the bench top models I'm aware of have a AC/DC motor and the floor models have a 120/220 v induction motor.
That doesn't mean a bench top won't do 75% of a small shop's needs. But, it may or may not. Shorter lengths than the outfeed table are about the limit. Here's why. A jointed surface will be straight and flat as long as it's supported along it's length on a straight and flat table. When the end of the material is longer, it will not be supported, droop down and now the cutter will start removing a different amount of material from the center. Starting with a curved board will result in ending with a curved board. This is why woodworkers ask "Why does my jointer make curved boards?"
See Concave Surface: http://www.newwoodworker.com/jntrprobfxs.html