It may be reasonable to speculate that UChicago is “harder”, just because it is more selective and has a reputation for academic intensity. Or not. Maybe UCLA professors feel more compelled to weed out weaker students with tougher grading.
On gradeinflation.com, the latest year represented for UChicago is 2006, with a GPA average of 3.35. For the same year, UCLA’s GPA average is 3.19.
http://www.gradeinflation.com/Chicago.html
http://www.gradeinflation.com/Ucla.html
How many data samples do these numbers represent? How close are STEM GPAs to the averages?
What are the averages just for students who enter both schools with similar standardized test scores?
Anyway, if it is true that one school’s average GPAs are lower, and if this is well known among random posters on College Confidential, then don’t you think graduate program admission committees also would be aware of this and take it into account?
So why not set aside this apparently unanswerable question and choose the college that otherwise best meets your needs and preferences? For me, if cost weren’t a big factor, the most important questions would focus on the quality of undergraduate education (not only in my intended major, which might change, but overall.)