People nowadays tend to prefer “express” and not really dwell much on one post, unless they really like reading. I think if your strength is advertising using catchy images, use Instagram, especially if your product is new in the market. I think for an advertisement to be effective, it has to catch your eye. And for me, you can actually do that using Instagram. You can use it as a “frontliner”. You can link your Instagram ad to Facebook, where you can put a more detailed information about the product.
An Instagram ad, usually is part of the feed, not like on Facebook, it is usually on the side, unless you pay higher for the ad, or you have your own page.
Another advantage of Instagram is that, not everyone advertise on Instagram yet, so you have less competition as compared to Facebook.
You can actually use both, since Facebook owns Instagram, which means, you can link your Instagram ads on Facebook and vice versa.