I can’t say with certainty whether or not X or Y would look sexier to a college admissions person, but here’s my two cents:
I personally really hate the concept of spending money to do fancy summer programs. You could easily learn the content online and the research experience can be learned through an internship or lab job which GIVES you money to do (what??? crazy I know). Additionally, the Summer Stem thingy is really new, and considering it costs 6.5k, I’m not convinced that it’d be a good way to spend all that money. I’m also skeptical of how much research knowledge they can impart over a computer screen that can’t be learned from a free online textbook. I feel that it would be a better use of your time to take courses tbh, or better yet, teach yourself Chinese or coding or something with online resources that cost zilch.
It’s totally up to you, and I’m sure the Summer Stem thing is a cool experience, I’m just not personally convinced of the investment-return ratio of expensive summer programs. EXAMPLE, my friend did COSMOS last year (it’s a similar summer program that costs thousands to learn stuff and get lab exp at a UC college), and I interned at a science museum. I got paid, he spent thousands of dollars. I got a lab job this year, he didn’t. See what I mean? Expensive summer programs don’t guarantee a good return on your money, and IMO, serve little purpose besides a spot on a college resume.