Wow, the coursework difference in those two programs is total, it's like 87 units difference. So you won't be able to do a two-year "second degree", you'll need to do another 3 years.
Looks to me like there's no way you'd be able to switch from one discipline to another for your master's with that much difference in coursework. A BEng in mech will know nearly nothing about electrical, you may as well be trying to do a master's in Buddhist theology.
With that in mind, in the work world there's a lot of interaction between mechanical and electrical engineers, so if you have 2 degrees you'll be at an advantage. Frankly, if you have the opportunity to do a second Eng degree, then it looks like a great idea to me, except for the extra tuition and losing another 3 years of your life. :-)
"I might as well finish it and get my degree" is a good and mature attitude to have. Though you may want to consider the value of that extra year of your life.
You should really talk to a departmental counselor though to make sure you're going to be able to do this. Engineering is a funny faculty, their rules are different. Maybe they can tell you whether you're better off finishing your 4th year mech, or just dropping back down to 2nd year elec.