People ask this before their first booking, usually because one is cheaper and they suspect the other is better. Neither is true in the way they expect.
What the springs change
A reformer adds resistance you can measure and a moving carriage that punishes a sloppy line — take the load off one side and the carriage tells you, immediately and in front of everyone. That feedback is why the reformer is easier to learn correctly and much harder to cheat.
A reformer will tell you immediately that your line is wrong. A mat lets you get away with it for another six weeks.
What mat work is still better at
Mat work uses your own weight, which makes it portable, cheaper and unforgiving in a different way: nothing assists you. It is the better place to build the breath and the ribs, and it is what we teach first in Foundations before anyone touches a spring.
If you can only do one, do the reformer and take the corrections. If you can do both, alternate them — most of the members here who have been coming for a year do exactly that.