Choose Langstrasse if it's CHF 100 cheaper per month, the commute to HB is 4 min shorter — more central and connected, there's a denser restaurant scene.
Choose Sihlfeld if it's noticeably quieter (55 vs 68 dB), it scores higher on safety (7 vs 5/10), it has better green-space access (4.5 vs 2.5/10).