This is really a personal choice. One isn't better than the other.
The Matwork is widely considered the hardest in the Pilates repertoire. It's just your body and the mat, there's no assistance. It is excellent for building fundamental, long-term strength. It is also the most accessible.
The Reformer is a moving carriage with springs to add assistance or resistance to your movement. With the addition of the springs, it functions as effective strength and endurance training, and provides support for rehabilitation or joint issues.
We love them both! But we recommend choosing the option which you are able to commit to at least once a week.