So, the sum of all this discussion is as such:
Tamron - Excellent value lens with great IQ (beats competition in the range), and is the cheapest 600mm lens. However, f/8 is required at 600mm to get good results. It is a zoom, so more flexible.
Canon 300 II + 2xTC III - Better IQ, faster, better build quality/weather sealing. Option to use native 300 f/2.8. Less flexible and 5x more expensive.
Get the Tamron if money is tight and/or wildlife isn't your thing and/or low light shooting is not a frequent event. Get the Canon combo if lowlight is more of an issue, you have plenty of cash to burn and you need a tank of a lens and the best IQ?
Did i miss anything? I like to have a summary at the end of these discussions!