Cost optimization - PCB Area vs. Double-sided load
In scenarios like this, I usually ask the fab/ CM to quote both. Even if the solution isn't totally routed, you can place components in an approximate location and change the board size to create an "A" and a "B" option. To them, it's just another quote.

Ultimately it comes down to the cost model of the company you're working with, and the size of your components/ overall solution (including how well it fits into a frame, etc.). If you have one large MCU and a bunch of tiny discretes, a 10% board size reduction may not matter in the face of labor costs for a second SMT process. 

However, if you have lots of large-ish components and a double sided board gives a massive size reduction, the fab can order fewer frames and potentially save you quite a bit.