I think both can get your son to wherever it is he wants to go (grad school, job, etc.), but they are different in feel--big state school vs. small private, UMD is very comprehensive with all sorts of majors, interests, etc. whereas MIT is (obviously) much more techy. Does he have a feeling about which one would fit him better? With the fact that you have money saved and he wouldn't be taking out loans to make up the different (or your family wouldn't be taking a vow of poverty or taking out a second mortgage or anything like that) I would probably say MIT -- it really is just such an amazing school -- but I don't think UMD would be a bad choice by any means.
CONGRATS to your kid for having two absolutely amazing options!