Sony A77 or Canon 5D3?

[gradebfilms]

Hey all, I'm new to this forum, although I've been watching Film riot for a lot longer. I recently got a Panasonic SD90, only because it was £240 for something worth a lot more. It's a great camera, but I'm quickly outgrowing it and I'm being somewhat hindered by it's lack of DSLR-type features (Quick manual focus/zoom etc.)
SO...to my point. I was looking at the Sony A77 and the Canon 5D3, and considering which camera would be best for short films on a budget. The only thing that confuses me is the debate between cropped/full frame sensors; what seems to be the only major difference between the two.
So, would it be worth spending the extra £2k or so on the 5D, or saving the money if there is no discernible difference. I've tried watching comparisons on Youtube, but as I'm sure you all know, Youtube's compression makes the entire thing a little confusing even further.
So yeah, any ideas would be greatly appreciated, thanks.

Also, I'd rather not consider another, lower spec camera, as I fear I'll just outgrow that one again just as quickly. Plus the A77 (maybe 5D?) is weather sealed, and would therefore be great to take on my expedition next year, rather than something I'd be worried about all-the-way through!