DD was told by a UIUC admissions rep that UIUC’s CS admission rate was 12%, but that was for incoming freshmen for the Fall 2019 and he was addressing a 100% in-state audience. Purdue’s CS acceptance rate is around 20%, but I believe that is overall and not broken down by IS or OOS.
Highly competitive any way you look at it.
Both are reach schools for CS, although it might be slightly easier (key word: slightly) to get in to UIUC doing a “CS + X” major, because those are housed outside of Grainger (engineering college).