Also 2 tuner units can record analog cable, the 4 tuner units can not. Doesn't apply to FIOS but something to keep in mind.
There are also other minor hardware difference like chip brands, etc... but nothing that really effects the functionality of the unit.
Get the 4 tuner unit if you can. Makes scheduling recordings a LOT simpler, plus it will be compatible with the Mini when that's released.
Developer for VideoReDo