Injection moulding and 3D printing solve different problems. The right choice usually comes down to one thing: how many parts you need.
The Core Difference
3D printing builds parts layer by layer with no tooling, so it's fast and cheap to start but slow and costly per part. Injection moulding needs an up-front tool, but then produces parts quickly and cheaply in volume. One is great for ones and twos; the other for hundreds and thousands.
When 3D Printing Wins
Prototypes, one-offs, very low quantities, and rapid design iterations. If you need a handful of parts or are still refining the design, 3D printing avoids tooling cost and delivers fast.
When Injection Moulding Wins
Volume production. Once you're making hundreds or thousands of parts, the low cost per part of injection moulding quickly outweighs the up-front tooling cost. You also get better material properties, surface finish and consistency. See injection moulding costs.
Using Both Together
Many projects use both: 3D printing to prototype and prove the design, then injection moulding for production. We're happy to advise on when to make the switch.
Sources & Further Reading
For independent background on the process, see the British Plastics Federation's guide to injection moulding and the overview on Wikipedia.