I used the Emperor 400 for about 8 years, it was a good reliable filter. However, if your water quality is already good, you won't gain anything by adding filtration, except spend a few cents more for electricity to run the additional filter, and a few dollars more each month for whatever kind of pads it uses.
I personally like a very simple tank, with slow moving water, instead of one cluttered with lots of different devices and rapidly moving water.
If you are satisfied with the service your Whisper has provided, you MIGHT consider a second identical Whisper, so you are just buying one type of filter pad.
If you think you really need some different kind of filter, consider a Hydro Sponge sitting inside the tank, they add a HUGE amount of biological potential, and if power goes off, the bacteria is still in your tank, where they are much less likely to die off than in an enclosed filter. Also, the Hydro Sponge cannot "lose siphon" and fail to restart when power is restored, the air pump just restarts and the bubbles start flowing again.
At present, I'm using nothing but Hydro Sponges, and my fish seem less stressed than when I had the Emperors, water quality is just as good, even though the current is much reduced.