Of the two choices ID75 through Ketchum is straighter, running mostly through a broad valley, with one summit (Gallena Pass)
Idaho DOT lists 4 pass on ID21. I don't find grade information.
https://maps.google.com/maps?q=Redfish+Lake,+Sawtooth+National+Forest,+Custer,+Idaho+83637&hl=en&ll=43.930046,-115.679992&spn=0.016103,0.03459&sll=47.272986,-120.882277&sspn=3.85381,8.85498&geocode=FRs2oQIdalEm-Q&hnear=Redfish+Lake&t=p&z=15&layer=c&cbll=43.930046,-115.679992&panoid=QTLGM0mXpkFKIRL8lVOQTQ&cbp=12,0,,0,0In this GM Streetview image, the google car is following a low boy trailer around a 20mph curve on the way up to Mores Creek Summit.
Ponderosa Pine Scenic Byway (21)Sawtooth Scenic Byway (75)