Both are good, but i think facebook is more easier than instagram for business. Becoz one of my friend is doing a small work with facebook and he one tried instagram, but did not get more response.
Instagram is the better option than Facebook,though Instagram is the part of Facebook but now people are moving more towards Instagram. So you should go with it.
Facebook and Instagram both are famous and popular social networks for any type of business. (small and big business)
It would be a better idea to conduct some research on your niche first and compare the overall traffic you are getting from both platforms individually. The channel with a bigger number of traffic should be chosen to run paid ads as it will have a better impact.