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...
Why a spare and not a backup? You can set a 2nd pump in the bilge with it's switch a little higher so it will only turn on if the primary fails or becomes overwhelmed. When the boat is unattended it is better to have it in the bilge rather than on a shelf.
I carried both Atwood Saharas that failed me back to West Marine, with little humor, and they obligingly credited me for both. I found in both cases the float switch was still working, but the pumps were no longer providing enough lift to push the water past the manual pump aft. After hooking up my backup little Tsunami it was spraying water several feet out the back of the discharge.