Hello David,
There is a book in the BAA shop entitled ‘Introduction to DSLR Astrophotography’ which may well be worth you acquiring, see: http://britastro.org/product/881
I have no experience with spectroscopy, but I would suspect the difference between the cameras cited would be very subtle. The Canon 60Da is optimised for astronomy as it has extended red sensititivity, but it costs about £750. The Canon data is here:
http://www.canon.co.uk/For_Home/Product_Finder/Cameras/Digital_SLR/EOS_60Da/
H-alpha regions will need far shorter exposure times with this camera, but if used for terrestrial subjects any red objects will come out far redder than their visual appearance. The normal cameras you mention both have 30 second and Bulb exposures so clearly can be used for astronomy.
Not sure what you mean by steered and unsteered? If you have a small equatorial mount, and there are some quite nifty ones around, such as the Vixen Polarie:
http://www.vixenoptics.com/mounts/polarie.html
…then you can take wide field shots with standard lenses that are several minutes in duration, quite easily.
However, for prime focus telescope work you need either a great equatorial mount or some form of guiding/autoguiding…. Without somehow guiding the exposure your shots will be limited to somewhere between 15 seconds and 2 minutes on the celestial equator, depending on whether your drive is poor or excellent, and what focal length you are working at. You can stack multiple short exposures using software such as Registax or Deep Sky Stacker. There are numerous handy videos on Youtube explaining how to use this type of software. For an astronomical telescope with a drive you need a camera to drawtube
adapter that will fit a Canon EOS bayonet mount, such as this:
http://www.harrisontelescopes.co.uk/acatalog/Canon_EOS_T_Ring.html
You may have a 1.25 or 2 inch telescope drawtube so you also need a compatible T adaptor to mate with the T-ring. For prime focus photography one problem often encountered with old Newtonian telescopes is not being able to rack the camera in close enough to achieve focus, but if you have achieved focus with a film SLR then you should not have a problem. As for eyepiece projection, well, this is usually used for lunar and planetary work, although most 21st century imagers use Barlows and Powermates for this purpose. For high resolution planetary imaging most imagers would not use a DSLR at all, but a high frame rate USB/Firewire camera that can shoot at up to 100 frames per second to give thousands of frames that can be aligned and stacked in Registax or Autostackert. An example of such a camera is the ASI 120MC, such as sold here (colour and mono versions are available):
There is also a BAA Observers workshop on September 20 which may be of interest if you are thinking about planetary imaging, although I’m not sure if DSLRs will be included:
http://britastro.org/meeting_render/3542
But I would definitely consider buying the aforementioned BAA book.
Other forum readers who have more experience with DSLRs may be able to give better advice about the specific Canon cameras you have mentioned.
Martin