we use Compressor to upres 720p to 1080i. i havent had a chance to try the Kona 3 yet, but i am definitely curious. the Compressor flow works great, albeit time consuming. the only caveat is that any noise or over exposure in the 720p shot will look more pronounced in the 1080i conversion. it has been my experience that 1080i (or 1080p24) originated footage definitely looks better than 720p. 720p is generally softer than 1080i, but if there is a lot of movement in the frame, its not really noticeable. since we overcrank our shots more than half the time, 720p is the main codec we use. we also mix our overcranked footage with 16mm film, so the slight softness vs. 1080i is preferred. since 720p takes up less P2 space and upconverts acceptably, we shoot it most of the time. we pretty much only use 1080i (or 1080p24) for scenics and interviews, where static sharpness is desired.
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btw, we shoot action sports documentaries (Snowboard, surf, motocross), thus the mixing of formats and frame rates.