Choose Langstrasse if it's CHF 150 cheaper per month, the commute to HB is 20 min shorter — more central and connected, there's a denser restaurant scene.
Choose Witikon if it's noticeably quieter (40 vs 68 dB), it scores higher on safety (9.5 vs 5/10), it has better green-space access (9 vs 2.5/10).