If you have the $1000 more to spend the 5D3 is great, but if not the 6D is a great alternative. We've been filming around moire issues for years with the 5D2 so saying the 6D isn't usable for shooting video is like saying a new Camry isn't drivable because it only goes just as fast as a Camry from a few years ago and not as fast as a new Lexus.
I've been using the 6D for video since it came out. My clients are happy with the results. Other shooters are constantly blown away by the low-light capabilities. If you absolutely have to shoot moire-inducing patterns regularly, and can't get around it by turning the in-camera sharpening off, tweaking focus, or pointing the camera somewhere else, you obviously know which camera to buy. Otherwise, if you can't figure out how to shoot a good video on the 6D spending the extra grand isn't going to help.
+1
Moire is potentially an issue on any camera, just so happens the roof tiles hit the magic pitch to annoy your sensor. I get the same issue on XDCAMHD when I film anybody with a foxtooth patterned garment. I remember having to butcher a PL filter and fit some stockings in place of the glass on the first firmware version of the VX1000. And I combat moire regularly on my own two cameras and anytime I;ve used a hired 5D2.
So it's not an issue unique to the 6D.
I treat everything I shoot on a DSLR. Bare minimum is a half pixel guassian blur over everything. I can use up to 1.125 on something really bad.
Things that help:
Keeping everything progressive. From your transcoding or interpretation, to your timeline field order to your output codec, to your DVD or BR burning. Its a progressive camera, make sure everything else in the chain is progressive. Don't a assume. Check.
Marvels DSLR plug-in can help if applied with care.
If you have a locked off shot you can also selectively apply blur using garbage matting.
If none of this makes sense, then I've over pitched it. You may have got a dud 6D. You probably didn't. And someday you'll hit a fabric or texture that does this to your 5D3.
Like titokane said, learn to shoot around it, via on camera settings and or how to fix it in post.