nyvz
Veteran
Any lens will be way sharper on the FS700+MB than a 5D3 when shooting video, especially wide shots (I have tested both setups). The 5D3 is a ~1620x910 camera vs. the FS700 (1920x1080). The FS700+MB with a Nanoflash looks excellent (422 100Mbit long GOP MPEG2) under challenging conditions (the internal 24Mbps AVCHD codec works well in most conditions). The 5D3 will get clean 422 out in April (resolution won't likely be increased, though (beyond doubling chroma)).
Are you talking about sensor readout resolution during video? I think the 5Dmk3 does capture a bit higher than that since it does seem that with 3x line skipping of a 5760 x 3840 sensor is actually 1920x1080. That of course is bayer data, so the 1080p YUV video is interpolated from that and certainly not as highly resolving as an camera that oversamples would be. Maybe you are thinking of the 7D which reads closer to ~1600x900 and is a bit softer than the 5Dmk2/3? Anyway, without oversampling at all, 5Dmk3 is fairly soft in its 1080p images. It is great for sharp 720p delivery though.
So the 5Dmk3 is at best capturing 1920x1080 line skipped from the bayer sensor whereas the FS700 is a 4k sensor originating images from a 3840x2160, so both cameras are delivering 1080p files, but the FS700 certainly will resolve a lot more detail.