Jack,
Personally, I can give you no compelling reason to buy a cape over a saddle. Most flies that you tie are between 18-14. Given that alone, you will have tremendous waste with a cape. When tying fully hackled catskill style dry flies, I have found that given a quality saddle, I can tie more flies in a shorter period of time. A few tiers that I know who tie commercially only tie with saddles for this reason. With a saddle, I can usually tie several flies with one feather. With a cape, it is basically one feather for one fly. There are exceptions to this, of course, but normally, I either have to pick out a number of feathers ahead of time or find a new one as I tie each fly. With a quality saddle, I have found barb size and density to be fairly consistent as you would also expect from a quality cape.
I think that it is more difficult to choose the right cape. Color certainly is important. Dyed vs. natural as well. I believe there are more choices available to the average tier in the capes vs. saddles from this respect. Maybe those commercial guys are hording all of the best natural saddles! Tiers also focus on the number of micro hackles a cape has. Of course this doesn't matter if you don't usually tie smaller than 18. What really matters is the number of hackles between 18-14, their length and their density. I have a number of Whiting silver label capes that I bought years ago that still have 2/3's of their hackles but no more 18-14 sizes. Choosing a saddle, in my opinion, takes alot of the guesswork and waste out of the process without sacrificing any significant quality. Of course, your mileage may vary.
Cheers, Mike