Hi Bryan,
I have the Canon 70-200mm f4L which I use for both Astro and "normal" photography!
I can recommend this lens for astro imaging where the need for IS is non existent. For daytime use the lack of IS at 200mm may be an issue, but it has never been for me so far.
The lens gives a beautiful flat field from edge to edge - no cropping off elongated stars due to coma. It also works very well with the Canon 1.4x extender giving a FL of 280mm@f5.6.
Please take a look at my signature gallery to view some pics taken with this lens (They are: Eta Complex, Triffid and Lagoon, Antares Region, Southern Cross and M7)
I reckon the Canon 70-300mm wouldn't be in the same class as the L lens for astro work, and I've read good reports about the Sigma f2.8.
Here's a great review site for these lenses...
http://www.photozone.de/canon-eos
All the best with your decision, I vote for the f4L! (biased moi!?)
Doug