With no bonus you get taxed on HK$58K/month if you don't join the rental reimbursement scheme, but on $41.8K/month if you do, for the same amount of gross income ($58K), so that's a no brainer. (Net income as a couple with no kids is $54954)
But if your bonus is defined as, say, 50% of salary and if you don't join the rental reimbursement scheme then your salary is deemed to be $58K, but if you do then it's deemed to be $38K, then your gross income out of the rental scheme would be $87K and you'd be taxed on all of it (leaving a net income as a married couple with no kids of $76.27K), whereas in the rental scheme your gross income would be $77K and you'd be taxed as if it were $62.7K (leaving a net income of $70.4K).
Tax calculator here: Salaries Tax Computation
If you're in a rental reimbursement scheme then you enter the total amount of salary and bonuses EXCLUDING housing allowance in the income box, and 10% of that amount in the "Value of Place of Residence provided" box (yes, this calculation is independent of the actual value of the housing allowance, provided that you spend at least as much as the allowance on housing). If you're not on a rental reimbursement scheme then you enter the total amount INCLUDING your housing allowance in the Income box and 0 in the "Value of Place of Residence" box