21 Stanley to Lowman crosses a broad pass, just a 500ft climb from Stanley, but otherwise follows a river.
21 south from there has a more serious 1800 ft climb before dropping toward Boise.
Banks-Lowman Rd, Cty 17 may have been a FS, but now it is nice paved. It continues to follow the river. At Banks it meets 55, a major highway from Boise to McCall. 55 leaves the Payette at Horeshoe Bend, followed by a modest pass. Other than potentially heavy weekend holiday traffic on 55, this should be the easier route, without has much of climb. Distance is nearly the same.