Usually, one model is cheaper, the other one drinks less fuel. So, depending on your strategy and you needs, you'll choose one or the other one.
Extreme example : the A140. in current GW3, you have 2 very different options :
- Motor-Sich Al-30, 1220NM, 680kg/h, 8 013 940 $
- Pratt & Whitney Canada PW127A, 1260NM, 570kg/H, 15 189 110 $
The fuel consuption is really different. But the purchasing cost is really different as well. If you are in a quick expansion phase, you'll favor the Motor-sich engines : you'll grow much faster with a limited amount of cash available. Even better : if you can't afford to buy, it will be dirt cheap to lease. OTOH, if you are an established company, needing to replace aging CASA 235/S340, and trying to optimize profitability, then the PW engines are far better on the long term. Assuming you've got enough cash to purchase the planes. Fuel is very costly, in A140's years.
This is an extreme example, though. Most of the time, differences are not impressive, and most players are simply buying the best engine, in most cases.