[sassan]

Then Gordon consider 100-400L as your best bet.

Well with L lenses you don't go wrong with two aspects; First quality is unmatched. In fact after using your first L lens, it is hard to add any non-L lens to your gadget.
Second; if it is not what you liked, with ebay at hand you are almost sure to get back what you paid for if lens is to be sell in mint condition say in less than a year time or only depreciating by 10% or so for next few years as no lens (Or for that matter photographic tools) keeps its value better than a Canon L lens. So you still have opportunity of going for the simga at the end if not happy with 100-400 that frankly I doubt. Not forget that with slower Canon lens, you will pay almost half of what you should pay for the sigma.