If you don't like them, go in tell them you were fitted for regular but the shop person recommended the Senior flex, you followed his recommendation and it did not work. They may replace them for you. I think the local shop I use would do so. It never hurts to try.
I have owned a Bag Boy 2 wheeled cart, Bag boy 4 wheeled cart and currently a Sun Mountain 3 wheeled cart. Both of the 3 wheeled carts are easier to push that the quad. My Sun Mountain bag is held nice and firm by the bag brackets.