Put simply, they aren't the same thing, though plenty of employers don't know the difference and assume they are. An M.Eng. can help you get a job, boost your GPA, and allow you to explore a major other than your undergrad. Because they aren't focused on PhD track students, there tends to be a lot of emphasis on getting students into industry positions. The administrators in my M.Eng. program worked tirelessly to find jobs for students, digging through their contacts, sending out plenty of job posting emails, sometimes catering assignments to give us specific job search-related skills. Basically everybody that wanted a job had one within a month after graduating, except a few stubborn jackasses (mainly me) who refused to settle for anything less than our dream research jobs.

The M.Eng. is kind of in limbo between undergrad and grad education, so you can really embrace graduate-level coursework and research if you take the initiative (read: worth the money), or you can use it as an extension of undergrad style assignments and projects (read: not as useful). If you're ultimately looking to get a PhD, you can use the M.Eng. to enhance your application and prove your mettle, but it's nowhere near as seemless a transition as M.S.->PhD. A few students in my M.Eng. program (myself included), are now either in on or applying to PhD programs. However, we had to go through the entire application process again, and none of us will be getting our PhDs from the same institution as the M.Eng. I absolutely have a vastly stronger PhD app than I would have without the M.Eng., but that is because I applied myself intensely, and I am still a year behind where I would be.

The other issue is that, since some employers don't know the difference between the degrees, sometimes (and I stress sometimes) an M.Eng. from can get you a closer look than an M.S. from . I've experienced this first-hand.

Bottom line:

M.S. is more prestigious, harder to get accepted into, probably better preparation overall, has the potential for funding, and is a much more direct track to a PhD
M.Eng. is more what you make of it. If you aren't going to take it seriously, you are probably wasting your money. All other things being equal (i.e., not considering a university's prestige), I would treat it more as a backup.