The reaction with KCN is much slower, and gives lower yield. See JOC 25(6), 877-879 (1960)
See below for a direct comparison:
1-Chlorobutane, NaCN, 140°C, 15 min, 93%
1-Chlorobutane, KCN, 120-140°C, 10h, 69%
2-Chlorobutane, NaCN, 120-145°C, 3h, 64%
2-CHlorobutane, KCN, 120-138°C, 24h, 42%