Mike. In general I like capes for traditional hackled flies and for parachutes saddle hackle. Jay is right about finding saddle hackle that large. I'm tying 50 dozen parachutes right now and have been pulling from 3 different saddles to try and get enough for 15 dozen 10's and 20 dozen 12's. Mind you these were ordered on the large side. The most common size I'm finding is 14.I'm using Whiting Farms.
Capes have a better selection of hackle sizes. Again 10's and 12's are tuff, but a lot more plentiful than on saddles .
You get a wider size variety with capes as compared to saddles but you might get lucky with a saddle, but I wouldn't count on any 10's if you want 18's and vise a versa. What flies are you tying?