1st, Bass Pro is not the place to shop for either unit.....you can do a lot better....Universal Mania for instance.
2nd, if the 798 is not the HD model, the only way to link the 2 units is with the interlink. It is old technology and I have seen connectivity problem reported with the newer software versions.
If it were me, I would consider selling that 798 and getting an 898....or you could get the 798hd si which has an ethernet connection....then link the units with ethernet.
The only difference between the 2 998 models is that the HD version has an LED backlit display that is brighter with better sunlight visibility. Your current 798 has better sunlight viewability that the older model 998.