While I'd say that Reddit makes things worse overall for the internet, Twitter is very clearly more damaging IRL. Through some terrible series of social errors that future historians will have to parse through, Twitter has become a nexus for journalists, celebrities, and even politicians to spew their soapbox idiocy through a megaphone - and this is regrettably substituted for actual, thoughtful discourse or critique. The 200-character limit ensures that context for one's statements are a thing of the past - the "new age" of digital debate consists entirely of picking and choosing which sentence-long snippet to quote out of context in order to damn one's opponents. Adding to this, of course, is the fact that Twitter's administrative staff are themselves ruthless ideologues who are hellbent on expunging any opinion, account, or even entire topic heretical to their chosen doctrine. The willful suppression of certain viewpoints in conjunction with the site's ubiquity creates what is an essence an echo chamber for a political sect along with a few appointed (blue checkmarked, that is) mouthpieces who espouse the vast majority of Twitter's visible content.I'll answer with this question: Which one make things worse irl?
The fact that a collection of posts from Twitter have been beamed into vast fields of space, potentially in the direction of other rational creatures, is a true cosmic tragedy.