Most Rangers come Mercury engines mounted and they seem to feature Mercs as their preferred outboard provider.
If your dealer is purchasing the motor directly from Yamaha, his wholesale cost of the motor may be substantially more than if the engine was purchased thru Ranger on volume discounts. For repowering, Mercury sells their motors to their individual dealers substantially cheaper than Yamaha dealers can purchase the same horsepower motor.
While both motors are great, see if your price on a Mercury ordered with the boat will be cheaper? The 75-90-115 hp motors are basically the same 359 pound motors with the 115hp costing about $1000 more than the 75 hp--and the 90 hp in the middle. Going with as large of a motor as your boat is rated for is really a better value and a more satisfied owner.
(Note: I'm a Yamaha F150 owner, and have had great service out of my Yamaha outboards since 1985.)