If you go with the WM Cub, the 95AX would certainly be the better choice for banner pulling. That Cub certainly would not need the 95AX for normal operation, but your consideration is more for the banner than the airplane.

Matter of fact, the banner size is going to be decided almost exclusively by the engine size and excess power, not by your airplane size alone. The idea that a bigger plane is better is based mostly on the power usually available to models by their size. The more power (ignoring how much is excess) available, the less effect a fixed load will impose. Of course, if you combine an oversized engine with any size model, you'll have excess power to use for other things.

For a couple of years, a number of us towed and piggy-backed gliders. Every tow plane that was used more than once or twice wound up having it's engine up-sized. We tried different props on a couple before up-sizing and they almost always obviously helped. The up-sizing helped of course, and selecting towing props for them also helped even more.