Robert, either lens is perfect for the job.
The 135mm is very very sharp, and fast (one of my favourite terrestrial lens).
Beautiful bokeh and colour rendition is second to none in it's class.
The 200mm is again superb, very sharp and perfect for astro and great for terrestrial too, do a search on some camera forums of photos taken using these two lenses, you'll be amazed.
They are both fantastic for astro, the 200mm great for comets as well and you can team them up with a 1.4x or 2x T.C and have more versatility.
I'd be hard pressed choosing between the two, I love both.
Attached is a SN screen shot of the fov they both give on the 20Da which has a similar fov to your 40D.
This is one supplier that I deal with, great service, never had a problem with them, they list the 200mm for $879, the 135mm for $1099.
Check out their site.
http://www.d-d-photographics.com/canoneos20dacc.htm
http://www.d-d-photographics.com/index.htm
