Answer:
"Ir a” is somewhat less frequent overall and found especially in casual speech, while the inflected future is found in all varieties especially formal writing. But both are commonly used, and of course easily understood. As other answers note, the difference is similar to English “will” vs. “going to”, although I will add that the inflected form in Spanish, I believe, has a bit more of a formal tone so “ir a” would be preferred, whereas English “will” is a bit more neutral