I am an engineer-to-physician guy.
1) Like it or not, AMCAS calculates GPA the same way for everyone. Your GPA will be compared to the other applicants.
Maybe someone on the ADCOM will say, "But he was an engineer, and that's a difficult major and is less prone to grade inflation. I say we give him the benefit on this one." Yeah, maybe. But at the end of the day your 3.5 is still a 3.5 and the A&S applicant with a 3.8 still has a 3.8 and you're counting on someone caring enough to notice the difference (and you're also counting on someone not being offended that their biochem degree was "easy" in comparison). Also, a lower GPA runs the risk of not making the first cut (where they are more likely to just look at raw numbers).
2) Getting a great GPA is really easy: work your butt off. It's a lot easier to study when you actual enjoy the material you're studying. It might be easier to do well in engineering if that's what you like simply because you will put in the time, where as doing a major just for the GPA will lead to a lot of time spent on SDN instead of studying.
Also, How many applicants for how many slots? Getting into medical school is a challenge, even for the extremely well qualified. What happens if after you've shadowed and volunteered in the hospital you realize you don't like the work? What do you do with your 3/4 finished "pre-med science" degree then? It's smart to have a back-up plan, and having a financially secure plan is even smarter.
Now....Engineering is good work. It exercises the mind and allows for some creativity. Usually 8-5, no weekends, holidays off kinda work. You'll miss less soccer games as an engineer. I know because I did it for 8 years before going to medical school. Being rejected probably would have been the better thing for me financially, as I would own my house at this point and not be saddled with loads of medical school debt, but medicine is also a good job. In about 12 more years, when I hit the break-even on the opportunity costs of leaving engineering, I'll be feeling really great about it. Let's also be clear....medicine is a job. They pay you because they can't find anyone qualified to do it for free. They pay you more because you have specialized knowledge and skill, but mostly they pay you more because work more and accept more liability risk.