The last one be listed is much taller and bigger, so while it may flex less, it might be to tall to fit the machine properly.
Looking at the picture, the way that you built the new carriage, it is more than likely the cause of the flex. If I am correct, you have a linear bearing on each side of the bed and the new carriage stradles the bed, kind of looks like a square C with the opening pointing down. Plus it is made of aluminum. All it has to do is flex a half a thou when doing hard materials and it will give a poor cut. On top of that, the aluminum won't dampen the vibrations like cast iron will do to the properties of each material.
Mike.
Warning: DIY CNC may cause extreme hair loss due to you pulling your hair out.