What situations are more suitable for each of these adjectives?
Are they similar in what they imply, or is "insulting" much stronger?
"Offensive" is pretty much something people identify as against morals, very inappropriate, or disrespectful.
Alex posted very offensive comments on other peoples' Facebook pages. They included several racial slurs.
That horror movie was deemed very offensive by many.
If you think the content of this video is offensive, leave now.
"Insulting" is something rude, insensitive and/or offensive, directed at another person or group of people. This emphasizes that the content is a direct attack against specific others.
Jamie said very insulting words to Ken, and that hurt his feelings a lot.
He wrote an insulting letter to the committee after he vehemently disagreed with their decision.