Hmm I think there no way to compare between them Because One is made for direct social interaction and connections and the other is for discussion of unique topics. Therefore they are two very different social platforms.
For me I prefer Reddit for the following reason:
The Reddit community is awesome. People aren't really selfish. Instagram revolves around everyone trying to get as many likes as possible just to feel good. Whilst Reddit is about a sense of togetherness. I think, getting a post hot that you've made on any subreddit means a lot more and better than getting thousands of likes on Instagram.