Progressive usually keys more easily because there are no fields which in an interlaced timeline blurs the edges making the key a bit more difficult. More than i or P, make sure your greenscreen is evenly lit. Use a waveform monitor to be sure, and shoot with a codec that has a 4:2:2 or better color space. Again, there are ways around all of this, but shooting it correctly at the beginning will make your edit life much easier.
John
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