The choice of engine is more than just a fuel consideration, the difference between the engine types could be 0-5% depending on the operation.
The CFM engine is lighter, burns less fuel lower (SFC of 0.2 lb/hr/lb lower), produces slightly more thrust in climb, however has a higher SFC 0.15 lb/hr/lb in cruise.
Depending on how the operator has their cost index set, this will effect the overall fuel burn for the flight.
Other considerations used by operators include technical support both from the manufacturer and line maintenance, existing fleet installations (i.e. CFM engines on A340 or 737), and flight crew (the IAE and CFM have different cockpit presentations).
The engine may also be owned, leased separately from the airframe, or leased with the airframe, the economics of these lease methods can blow any fuel saving, and may in mean a level of compensation to the operator is delays in schedule are caused by engine related delays.
Last I looked all the lease A320 aircraft that were available presently on the market are all V2500 powered, i.e. no CFM powered airframes are without a home. This may an indication of customer preference.
Engine type CFMI CFM 56 IAE V2500
Engine model 5A1 A1
Thrust (lb) 25000 25000
Flat rating (°C) 30.0 30.0
Bypass ratio 6.00 5.40
Pressure ratio 26.50 29.40
Mass flow (lb/s) 852 781
SFC (lb/hr/lb) 0.33 0.35
CLIMB
Max. thrust (lb) 5670 5620
CRUISE
Altitude (ft) 35000 35000
Sfc lb/hr/lb 0.596 0.581
Basic eng. wt. (lb) 4860 5210
LAYOUT
Number of shafts 2 2
Compressor 1+3LP 9HP 1+4LP 10HP
Turbine 1HP 4LP 2HP 5LP
IAE has further information on their web site
www.v2500.com
Regards
Z