SO is a Cornell grad and D is at Penn in CS and was admitted to both.
Penn and Cornell are both excellent CS programs. CS is the most popular engineering major at Penn. Both produce grads beginning with average starting salaries of about $105,000 based on their most recent career surveys. I could not find similar data for Northwestern.
D chose Penn but a different student could easily make a different decision. Things she liked were the One Penn policy that has allowed her to explore other schools at Penn, which she has used (she finished a math minor and needs one more course for a stat minor). Penn also has good research opportunities. She spent her freshman summer working as a research assistant for a Wharton professor. She prefers the city experience of Penn to Ithaca. She also likes to visit NYC which is only an hour away.
There are also many clubs and activities at Penn that she was interested in, such as SWE, Women in Computer Science, Tech Trek, Hack for Impact, Penn Apps, and Femme Hacks to name a few that come to mind. I am sure that Cornell has many too, but I am not as familiar with them.
She also explored the dual degree and special program opportunities at Penn.