Okay, a little food for thought as I'm in the process of deciding the transition myself. I'm coming from a full-frame Nikon D750 and I have all the f2.8 lenses (14-24, 24-70, 70-200). I've pored over this issue for the past month but a lot of the revelations also apply to the Canon system.
- weight advantages for the A7 series tends to disappear when you get to the f2.8 lenses. For the most part, they are just as heavy and more expensive than their Canikon counterparts. You can't really get around the physics of optics when designing lenses of this size. They are all generally that big and heavy because of the fixed aperture.
- weight advantages DO become more evident with primes and smaller aperture zooms (e.g. f4), mainly because the body makes up a larger portion of the total weight with lighter lenses. The ~260 gram advantage of the body will be more evident with your lighter primes (in which, I believe Sony has slightly lighter primes).
- in-body image stabilization (IBIS) of the A7 II is a huge boon. I also use a m4/3 body with IBIS and it's incredible for getting shots at low shutter speeds for still subjects. This point is kind of moot though if you're only ever going to shoot with the f2.8 zooms because they all have built-in IS. If you are going to shoot with primes without IS or adapt lenses of other systems... this advantage cannot be understated for hand-held shooting or video.
- Price is another thing to consider, Sony lenses are generally significantly more expensive than Canon's. The 70-200 f2.8 is a good example. Canon has a top notch 70-200 f2.8 but is even cheaper than Nikon's. Floating around HK$10,000. Given the price the Sony put on the 24-70 f2.8, the 70-200 will likely be much more than Canon's. Keep in mind that you're not just choosing a camera body, but you're buying into a complete lens ecosystem.
- Lastly, I would point out that the Canon 5d MIII is growing a little long in the tooth. It's a widely used camera but to be fair, it is now a 4 year old camera that is long overdue for an update and should probably see an update within the next year or so (though 5D users have been holding their breath for a while now). The features of the A7 II is just more up to speed with current generations of cameras.
Now with that said, I've decided for myself that the f2.8s might not just be worth the weight and instead I want to shoot as light as possible using a full-frame body and light f4 zooms and fast primes. Throw in IBIS and I'm really sold on the A7 system. I think for yourself, you'll need to think about what lenses you plan on shooting with (f2.8 zooms, f4 zooms, primes, adapted lenses) and what your priorities are in terms of weight and cost. I think really, these two systems are neck in neck in terms of IQ.
Finally, a shameless request. I'm also an expat based in Hong Kong and looking to make my A7 purchase asap..... any way I can hop on that 25-30% discount?? I'd be hugely grateful! And wouldn't mind helping you through the buying/deciding process.