Best HMC40 video format to convert to SD
 

We are using Sony VX2100s currently for wedding work, but just got a Pany HMC40 to work with, starting a slow transition to HD. I've been shooting in the native 1080i mode, but was wondering if it would be better to shoot in 720p mode for converting to SD? We are using the MainConcept transcoder to covert the files to SD for use in Vegas 6 (we'll upgrade to 9 when we upgrade the computer, still using a Pentium D machine). We are transcoding to 4:3 format. Any thoughts on which would be the best format to shoot the Panasonic in with this workflow?

Thanks,
Robert