As always, comparing L lenses to "normal" lenses is a process that can lead to something called "L-disease".
That said, I would definitely go for the L lens. Not because it's supposed to be better, but because it is. Whatever reviews may tell you (I especially like The Digital Picture, which you have links to in Poldie answer - but some may think it's too biased), the L lens will always have superior build quality, better bokeh, better sharpness and better contrast than a "normal" lens. This is going to be tough to see at, say, f/8, but at f/4 everything would be almost as clear as at f/8 in the L lens, while on the others maybe not (to see this, on the Digital Picture, there are tons of sharpness charts where you can compare your lenses at almost all apertures).
In conclusion, as much as the 18-135 can be a good general purpose lens (wider range, probably lighter weight than the 24-105 L), that little L really makes the difference in the long run, especially in terms of build quality. Again, one might argue that a lens like the EF-S 17-55 f/2.8 IS is a great lens and should be made L. But that's a whole new discussion.
(In general, for lenses, the more you spend, the more you get)
