The verdict
If your buyers research your category in identifiable communities, pick Reddit Ads. It suits technical and niche topics, and you can reach buyers when they are asking the question you answer.
If your buyers follow live conversations and you can react quickly, pick X Ads. It suits a B2B brand or media team with timely content, such as a launch or an event.
If both apply, run a capped test on each for the same goal. Reddit asks more of your writing and X asks more of your speed. Reach on X has shifted over the years, so start small. Neither is usually a first paid channel.