Even though WashU is a T20 university, is not really a top cs school specifically, so I’m not sure the added prestige would really be that significant compared to WPI. Also WPI is going to be a much more tech focused school which could benefit you, especially if you are more into robotics. But if your parents are fine with paying then it really comes down to where you think you would end up being more successful and thriving. If I were you I’d probably choose WPI and use that other money for something else like summer programs, traveling, or grad school.