If your budget allows go for the 135mm f2L lens then send it to me!!!!
That is one of THE widefield astrophotography lenses.
Take a look at the pics on my photobucket page (link in signature below)
to see how different lenses frame specific deep sky objects.
As you'll see, a 135mm lens would nicely frame the LMC, Eta Carina Nebula etc in a wide field shot.
The lower the f number, the quicker the camera will record data.
eg a 30sec exp @ f2 will give you more stars and detail in your image than a 30sec exp @ f4. A fast lens allows shorter exposures which minimize any tracking errors in your mount too.
With the L lenses you could then use a 2x or 1.4x Canon extender to turn a 135mm lens into a 270 or189mm lens without degredation of quality.
Cheers
Doug
