So I'm wondering if there is much more that the 70D would have to offer cf the G6 (image quality and performance for stills and video) that would more that justify the additional size and weight?
I shoot video (and stills) on a Canon T4i/650D (and sometimes use the 18-135mm STM lens you mentioned) and a Panasonic GH2 (which has the same sensor as the G6, except that the G6's firmware doesn't support multi-aspect modes). I'm planning to upgrade to the Canon 70D for video, but when I shoot video, it usually involves a tripod/monopod/glidecam, dual system audio, lights, etc. so camera/lens size is not a concern. When camera/lens size is a concern, I will usually use my Olympus E-PL5. So for me
, I would say, no, the 70D does not offer enough of an advantage in image quality to offset the additional size and weight.
the canon system with the whole variety of 3rd party lenses have much more variety at a cheaper price point cf panasonic m3 lens system.
True. But remember that Canon EF lenses are designed to cover a full frame sensor, so they tend to be much larger than equivalent m4/3 lenses. (Also true of EF-S/APS-C lenses (like your current e-mount lenses), just not to the same extent.) And since small size is a concern of yours... (Also, for video, note that Canon doesn't currently have many STM lenses.)
On the other hand, since the vast majority of m4/3 lenses are focus-by-wire, they can't really be used on your VG10, while EF lenses can (although you will not be able to adjust aperture, only focus) with an adapter.
So as usual, it seems to me to come down to the lenses. What do you need/want? Does m4/3 have them? Are they within your budget?
P.S. re: "the new 14-145 f3.5 lens" - if you mean the 14-45mm lens, I'd note that it's f3.5-5.6, not a constant f3.5