Short answer: post to both, but lead with the one that fits your content and audience. The same vertical video usually works on both, so covering both is mostly a scheduling problem, not a creative one.
TikTok — built for discovery
- The For You feed surfaces content to people who don't follow you, so new creators can reach far beyond their follower count.
- Trends move fast, and native, casual video tends to win over polished ads.
- Great for going from zero to an audience quickly.
Instagram — built for relationships (and selling)
- Reels get discovery too, but Instagram's strength is depth: Stories, DMs, and a profile that doubles as a storefront.
- Often closer to where buying happens — link in bio, product tags, brand deals.
- A warmer home for an audience you've already started to build.
The pragmatic play
Most small creators shouldn't pick one — they should make once and post to both:
- Shoot one vertical 9:16 video.
- In Compose, select Instagram and TikTok (and YouTube Shorts while you're at it) to cross-post.
- Tweak the caption per platform if you want, then schedule.
That turns "which platform?" into a five-minute scheduling step. See Schedule your first post and How often should you post?.