Mixing HDSLR with HVX200a

I know there was a post about this last fall, but wasn't sure if my question would be answered if posting on something that far back.

The post I came across said if you want to mix canon 5D footage with panasonic hvx200a footage, you need to shoot in 720p.

Can I ask why shoot in 720p? I have an hvx200a and hpx170 and was hoping to mix them with 5d footage for a wedding. I wanted to shoot 1080p for blu ray. Is 720 better because of the rectangular pixels? How would 720p look on blu ray?