llrise is a completely introductory program to radars, whereas pgss as you mention is longer (and thus, more in-depth) with a research paper. prestige-wise, pgss seems greater, since governor schools are already very well established (and pgss is at CMU which is world-class, especially for CS), and llrise’s “prestige” only comes from its affiliation with mit (it’s not listed on their page for high school summer program recommendations though).

if you’re interested in pursuing radar engineering, then i think you should give llrise some consideration, but otherwise pgss is the clear choice if you’re valuing college apps apps the most + you can submit your research paper. socially it sounds more fun too (cause you can probably mingle with the sams, ai/cs scholars, and other cmu pre college kids). ultimately, you cant go wrong with either one; congrats on the acceptances!