I finally after much umming and arring decided on getting the Canon 300mm F4L IS + 1.4 TC.
I decided on this setup over the 100-400mm of the images I saw of both, the 300 + TC seemed just a bit sharper, and if I take the TC off, I have a great little 300mm F4 prime lense.
The only problem is that I can't use it until Feb 28. :cry:
Once I get it I intend to head straight to my favourite birding location as soon as I get the lense to see how it goes. So I've posted a fun little poll for people to try and guess which bird will be my first posted bird with the new lense.