[ninetto makavejev]"Should I interpret the footage as "separate fields" or export as an interlaced file"
if you choose not to separate fields, you should render as progressive to maintain the fields that are already present with in the frames.
if you choose to separate fields, you can render progressive or interlaced -- if you choose to render progressive, you should enable the preserve edges option to help make hard edges a little smoother. if you notice some fluttering around the edges during play back, try enabling frame blending (frame mix) for the footage. that will use the data from the second field and usually helps to smooth that out.
there are situations where you'd have to separate fields.
if you have interlaced footage, you have to choose to separate fields if you are going to manipulate the vertical position, vertical scale, or rotation of the footage in ae or you will get terrible artifacting, no matter how you output the file (interlaced or progressive).
likewise, if you are keying, when you shrink the key, you will then remove fields around the edges, which may produce somewhat strange movement, as the material within the key will be interalced, but the edges won't be.
if you are adding motion graphics to the footage, then you should also choose to separate fields, or your footage will be interlaced and you motion graphics will not... in some cases this may not be a big deal, in others it may look strange.
as i mentioned before, once separate fields is enabled, you can choose to render either interlace or progressive.
if you are not doing anything like that, say you're only doing color correction, then you could choose not to separate fields, and then render as progressive (which will maintain each frame the way it is, with interlacing) and everything will be fine.
if it is going back to avid, when they import it, they need to know that the field order is the same as what they gave you.