1st thing I can tel you is... the 2.8 w/TC is a beast 70-200=3.28 lb + 2x TC=11.46 oz so you're talking about 4lbs the 100-400mm weighs in at just 3.04 lbs I'd say the IQ is better on the 100-400mm over the combo, but not by tons.I'd say the IQ on the 100-400mm beats the combo by a good amount out to 370mm though.
If you're only doing wildlife the dust pump wins hands down if you want a lens to do closer work, portraits and on a rare occasion wildlife, and can't justify,(or afford both) consider the 2.8 TC combo. I started using the combo, but ended up getting the 100-400mm and haven't looked back.
and try and get the IS if you're adding the TC... the longer the focal length the more you'll notice any small vibration you take close shots with a 50mm so IS not all that critical, but when you move from a portrait distance(10 ft.) to a whitetail deer at 100 yds, that small vibration magnifies to a shake.
I found a strange thing... I was using them on a 7D and the IQ on the 70-200mm + 1.4X TC was significantly softer than the 2X combo(I have the mark III versions of the TCs and my 70-200mmL IS is version 1(not sure if that is why
hope this helps