Is ECO or Sport mode more fuel friendly on motorway Mercedes Benz CLA 220d 2018?

Truthfully you won't notice much difference between the two modes while on the motorway.  The biggest difference while driving is how you use the accelerator pedal.

But ECO mode does help save a little fuel while accelerating as it has the transmission upshift earlier compared to Sport mode

What do you SUPPOSE ECO stands for???  SPORT MODE makes the TRANS shifting LONGER to talked advantage of HIGHER ENGINE SPEEDS! >>>use LIQUI MOLY DIESEL PURGE as a TUNE up in a CAN for DIESEL engines!0 ON the HIGHWAY the engien RPMS are the exact SAME for ANY MODE! TRY it YOURESELF! The COM FORT MODE tones DOWN the SPRT STRUTS to make the ride more FORGIVING and LESS tight!  YOU are ON lucky CAMPER! BYE NOW! 

Sport mode wastes gas but might be more practical in city driving at lower overall speeds.

Eco mode saves gas.  Eco=Economy.

ECO. Sport mode usually just changes the shift pattern so the tranny stays in each lower gear a bit longer until RPM's reach a higher threshold before shifting to the next higher gear. Sport mode wastes gas but might be more practical in city driving at lower overall speeds.

of course its eco mode ...sports mode holds the gears to a higher revs before changing 

Sales person is an idiot. You are correct; use ECO mode. It will shift gears earlier, to maximize fuel economy.

Sales people don't always have the full information on the products that they are trying to sell. This is especially true if they have a large and varied inventory of products. If you want to know which mode the manufacturer recommends, you should actually READ the owner's manual that came with the car.