In one case, you will pay fix amount every month plus insurance which will be decided at lease start.
In other case, your monthly instalment would be calculated at the start of each year of lease. In simple terms, if interest rates go down, you would benefit. If they go up, you will pay additional.
I suggest that you go for fixed rate. interest rates in Pakistan are already at the record lowest and there is less chance of them going down any further so there is no visible benefit in sight. Plus if you go for fix rate, you will know how much to pay every month and you will get used to it in a few months.
At the end of the day, 19 20 ka he fark ha.