I've worked in security for years and I've got quite a few colleagues who do semi-pro photography, so I hear about both phones pretty regularly. The thing with the S25 Ultra is it's clearly got the edge in low light thanks to its bigger sensor and really aggressive computational processing - your photos will be brighter but sometimes a bit too processed, you know. The iPhone on the other hand does something more natural, a bit less noise but also a bit less detail when it's really dark.
For Instagram and TikTok specifically, the S25 Ultra's gonna give you more saturated, punchy colors that'll pop better on the feed, whereas the iPhone will be more subtle and smooth. Honestly if it's really low-light photography you're after, the Samsung will serve you better, but you gotta test both in hand because the rendering really depends on your personal preferences - some people think Samsung's processing is too aggressive, others love it.