Which specific 70-200mm are you referring to? Canon has 5 on the market.
Personally the only 70-200mm that I'd consider as suitable comparison is the 70-200mm f2.8 IS L MII. That lens (the newest and also most expensive) has improved optics over the original and is an outstanding lens for its 70-200mm range. Furthermore it will take a 2*TC and deliver image quality on par with a good copy of the 100-400mm L (with both also showing marked improvement in optical quality after f7.1/8).
The other 70-200mm f2.8 lenses will mount a 2*TC, but are not as good - I owned the previous version of the lens and the 2*TC was something I used only once = whereas the MII I'm willing and able to use it all the time without any worries.
On the other hand the 70-200mm f2.8 MII is heaver than the 100-400mm and more expensive and also has slightly slower AF (when using the 2*TC). If its pure reach you want the 100-400mm would be a good option to consider - also you've the good quality 300mm f4 IS L (+1.4TC) and the 400mm f5.6 L. The 70-200mm f2.8 IS L MII is suitable only if you also want a sharp, fast focusing and excellent 70-200mm lens in its own right.