Threshold for Foreign Housing Exclusion in 2018 was USD 45.55 per day. I.e if you lived outside of US for full tax year (same as calendar year), then you multiply $45.55 x 365 = $16,624. Any housing expense beyond this number is your housing exclusion capped at country rate. Fortunately for Hong Kong, daily rate is pretty high (approx $ 313 per day in 2018 or a total of $114,300).
HKD 25,000 per month = HKD 300k per year = USD 38,593 (at today's rate). Your exclusion will be $38.5k - $16.6k = $22k (rounded).
Your total salary excluded from US taxes will be USD 103,900 (2018 limit) + USD 22k = $126k (rounded).
you can do the math with HKD 18k per month and see which one will be beneficial to reducing your US taxable income, and therefore tax you will owe to US.
Also, outside of tax consideration, is $25k unit more convenient vs the $18k unit?