Should you opt for a furnished office space in Mumbai or an unfurnished office space. The difference between the surely here is in the rent per sq.ft and also the type of furniture you are already getting without you having to get the entire thing done and get into cost over runs etc.
Typically, this breaks down to Capex or Opex, Capex being a Capital Expense which mean eroding your capital into a fixed asset or a furniture which has a high depreciating value and if you not required it will further be of no use and sometimes you have to pay to get it out of your office.
The fully furnished office on the other hand is an OPEX, which means you are paying the rent/license fees for the office but there is a tax write off you get on the same.
So ideally, if you are looking for an office for a short term of 12 to 36 months, ideally opt for a fully furnished office and you should ideally agree to pay to the owner a higher rent for fitting out the office space.
Cost of fit-outs is expensive and in a city like Mumbai it becomes extremely expensive to get it done and needs a certain amount of expertise and experience.
Also, keep in mind if you are looking for an empty office you may be paying rent at the same time unless you get a few days or weeks or a month or more as rent free period from your property owner.