Ariba is a good product to learn to compliment SAP MM. As a note, Ariba is a Cloud product so it will typically be less resource intensive to implement and support.
No-one can tell you whether it will be a good career move for you. However, I always see developing skills and expanding knowledge can only be but a good thing. You might find yourself at a client that is considering implementing Ariba and if you have the knowledge then you've put yourself in a prime position. More so, if you learn the integration between to the two products 🙂