Lets start with the end immediately. I think the answer is both.:)
Instagram and Pinterest are the latest hype of the social media. Both of them are very popular with fast growing communities. Some emphasizes the potencial of them as a marketing tool but I think Instagram and Pinterest are rather oases that are not poluted with advertisements and junk.
Pinterest is a web based site where you can share your favorite contents sorting them into different folders that they call boards. The application inserts photos from a URL into a relatively large frame that will be visible to your followers or based on a the given tags to the public also. Publishing something is called pining there, every entry of yours will be a pin that can be liked, commented or repined (shared). Interesting or useful contents can be very easly viral like this.
Since pinterest includes the URL of the website where the photo was originally found this tool has a real marketing potencial. You also can check, what other contents (photos) where shared from the very same blog. But a pin never contains anything else but a photo, the short description you type there and the url of the the site.
I find Pinterest very impressive as looking at the screen all you will see is a full screen of photos that are organized into boards or thematized with channels. An eyecandy.
Instagram is a smart phone application. This is the newest generation. They even didn't bother with a PC version, Instagram is available only on smartphones or tablets. Here you can share only photos that can be hashtagged. Impressively viral, after the first few days I have received more than 100 followers. The most amazing is that Instagram has supercool effect. I was never satisfied with the quality of iPhone photos. With the effects of Instagram you can make miracle. They created a new style of urban art with this application.
I love both application. It boosts creativity and with the help of the social media sites this creativity flows everywhere.