Both save the date cards and wedding invitations are important things to think about when planning your wedding. They are both used to communicate the date of your wedding, however, they're not the same thing. So, rather than either-or, you might want to consider sending both.

What's the difference?

The difference between them is that save the date cards are generally used as a somewhat informal advanced notice of the date you've chosen for your wedding. Naturally, you want to give your intended guests as much time as possible to ensure they're able to have the day free to attend. Work and other family commitments can get in the way, so it's the polite thing to do to let people know well in advance of the date you intend to marry.
What's the Difference between a Save the Date Card and an Invitation? - Formal Monogram Save the Date Cards
A wedding invitation, on the other hand, is a formalised notification that the wedding is impending and that you have been invited. An invitation will normally contain more information than a save the date card. A save the date card will generally contain the recipients' names and the date of the wedding. An invitation can have more details, such as information about the wedding venue, dress code, or details about any gift arrangements.

Which should you send?

Why not send both? You don't need to choose one or the other, because save the date cards are particularly easy to create when you use Tree of Hearts.
What's the Difference between a Save the Date Card and an Invitation? - Minimalist Typography Save the Date Magnets
They're affordable, too, and a very practical way of ensuring that all your desired guests have enough notice to clear their schedules so they can attend your special day.
Wedding invitations are considered more of a necessity than save the date cards, however, so if you're only choosing one or the other, make sure it's the invitations.
What's the Difference between a Save the Date Card and an Invitation? - Navy Jewel Flowers - Tri Fold Wedding Invitation & RSVP
It's worth considering sending save the date cards too, as they're so simple and can help you to save headaches later on with guests not being able to attend.

Choosing the right designs

It's important to choose the right designs, language, and layout for both your save the date cards and wedding invitations. So, make sure you choose a specialist supplier to provide them.
What's the Difference between a Save the Date Card and an Invitation? - Rustic Wedding Lace - Boutique Wedding Invitation & RSVP
Tree of Hearts has many different designs for you to choose from, allowing you to make the perfect choices for your wedding.