My 2 cents is that if you have been pretty happy with your 550d you will be more than happy with the 6d for a while, not to mention your bank account will agree. It is also a more familiar system. And while the A7's might have a slimmer profile, there isn't that much of a difference in size once you get a lens on there. The A7 would be a big jump in quality for the most part, but do you "need" that jump? The 6d is still a decent upgrade. You could invest the extra money in another lens you want to try also. Not to mention, we are at the point where you could transition to a quality canon mirrorless at a similar price point rather than completely jump ship to a Sony