From my personal experience (have been on TT for about a year 290k followers) (Ig for a few years about 100k over a few accounts) , Instagram is better just because of the amount of problems TikTok has with you trying to drive traffic to your monetisation techniques. For example:
If i make a post on TikTok with a cta to my bio, even the retention rate and engagement is awesome, they wont give me me any reach. On instagram however, i put ctas, both in video and description and the video still reaches over 1m views most of the time. Also i believe having a high reall following on instagram is much more valuable then Tiktok imo.