What mat pilates actually is
Mat pilates is the original method: your body against gravity, no machine. It's harder than it looks, because there's nothing to help you cheat — your core has to do the work the springs would otherwise share. It is the purest test of control there is.
What the reformer adds
The reformer is a sliding carriage on adjustable springs. Counterintuitively, those springs can make a movement easier or harder: they support you where you're weak and resist you where you're strong. That's why the reformer scales so well from a first class to a tenth year.
Which is better for beginners?
For most people, the reformer — not because it's easier, but because the machine gives you feedback. The carriage tells you immediately when you've lost alignment. On the mat, that same mistake stays invisible until an instructor catches it.
Why choosing is the wrong question
The two train the same thing from opposite directions: the reformer teaches you the shape, the mat proves you own it. Studios usually force the choice because they only have one room. SĀMA doesn't — both live on one membership, along with hot room, barre, yoga, aerial and recovery.
The short version
Start where you're curious. Just don't let a studio's equipment decide your practice for you.