I find the best way to judge these scopes for their astrophotography potential would be to look at actual results from people that have used them, and I think the WO website has a few samples in their gallery you could see. If not, search around, I've seen a few people get good results out of the SD doublet, although you will probably want the field flattener with it if you image with a digital SLR.
Also, you can read a review comparing the 66SD with the 66 Triplet at
cloudynights. The ED model is a petzvel design (4 pieces of glass) and is only rated as semi-apo.