They are 2 very different types of dining establishments.
Biergarten - you will need a ticket to enter Epcot. The entertainment is not constant but there is an organized show at scheduled times. The food service is buffet style but very tasty. If you party is less than 8, you may be seated with strangers at your table. Depending on your personality, that will either be exciting or strange. The offerings are a bit of American food and a bit of German fare.
Whispering Canyon - located off the lobby of the Wilderness Lodge. Here your table is you and your parties own table. The waiters and waitresses all have a schtick they perform to different degrees. If you are not into the performance art, in my experience, if you don't play into it, they get the hint and just serve your food. The food here is also very tasty and served either in platters for family style dining or ala carte.
Hope this helps!