Well first of all, this issue has been discussed loads on these boards, but no matter, let's start another one
I'm actually in the same boat, and my advice to you, first of all is to go and test them for yourself in a store (I am going tonight to a local store to do just this).
I would love to have the build quality of the L, and from what I gather, it is stupidly sharp, but then so is the 70-300. The extra reach for me is important as my intention for this lens is for travel and wildlife.
So really, figure out what you want to do with the lens, and go and test them out side by side.
It shouldn't be a hard choice if you really think about it.