The RWD/FWD vs AWD debate is different depending on what car you are referring too.
Some AWD systems are RWD most of the time while others are FWD most of the time. Some AWDs are 50/50 front and rear all the time.
Cars that are true AWD and 50/50 split experience some understeering which is what makes people think RWDs handle better. Handling is subjective, it depends if you are referring to lap times or referring to the driving characteristics (turn in, stability during turns, oversteer/understeer) or if you are just talking about pure driving enjoyment.
The 4Matic in the CLA is most likely front wheel drive biased. So its basically a FWD unless the car feels like the rear wheels need power to help it through. Generally when traction in the front is lost during turn in (understeer) it will send a certain percentage of power to the rear to help it through the turn.
Personally I think AWD (RWD or FWD bias) is a better rounded car over a RWD or a FWD. But some RWDs will still perform better vs a FWD bias AWD vehicle.
Keep in mind the base CLA being FWD doesn't mean it will be a bad handling car. There are some FWD vehicles that exhibit very little understeer through enginnering and design of the chassis and suspension.