UMD 2017 Engineering Outcomes: http://www.careerengr.umd.edu/sites/default/files/documents/2017%20Full-time%20Salaries.pdf
CMU: https://www.cmu.edu/career/documents/2017_one_pagers/cit/2017_1-Pager_ECE_All_Real_Final.pdf
https://www.cmu.edu/career/documents/2017_one_pagers/cit/2017_1-Pager_MEG_All.pdf
So the mean for a typical CompE student at CMU is about $10-15k higher. However, the difference between MechE grads is neglible. So, for CompE and CS CMU has a significant enough return that it likely would be worth it, but the same doesn’t hold for other specializations within engineering. However, it’s clear that CMU is a clear step up prestige wise (for computer-related stuff) and the higher starting salaries reflect this. It is worth noting that since UMD is a public school, the salaries are naturally dragged down a little. For example, GA Tech (which is ranked higher than CMU for Engineering) grads only start around $5k higher than UMD grads, despite being ranked much, much, higher. Since you got into CMU, there’s a good chance you would be among the top students at UMD.
Personally, I would say go with CMU is the difference is relatively close; maybe something like $20,000 a year? Otherwise UMD is simply too good of a deal to pass up, and you could probably get into CMU for grad school anyway.