I dont think the all in one units are necessarily better than seperate units, but most of the newer all in one units are using water sensing solid state "float switches" now as opposed to an actual switch that floats, so that part is far more reliable. If you buy a seperate water sensing "float switch", and wire it to a stand alone pump, i think you will acheive the same exact reliability, assuming you seal the electrical connections properly.
I chose an all in one model with a water sensing switch vs seperate pump and water sensing switch only because it fit much easier than installing two seperate units would have... It is probably cheaper up front for the combo unit also, but may be more expensive in the long run as the switch will outlast the pump 2 or 3 to one...