I can probably console myself with the 70-300L being more "walkaround", which I do a lot when birdhunting. Although i would very much like a 300/2.8, and if that 300/2.8 comes in black with an extra 120-299mm as well...
You bet the 70-300L is more "walkaround". Compare the weight!
EF 300 f/2.8II 2400g
Only Mr Muscles would consider the Sigma as a walkaround. Wouldn't it be cool if the Sigma weighed in at a less punishing weight?