Streams for me.
My knowledge of codecs is far too limited to explain why. When you say the client seeks to end explicitly are you referring to the client ending the download and playing from there or something else? If the former, the stream doesn't seem to effect the download process. Although a download progress indicator as a part of status during playback would be nice.
For those wondering, streamed videos play the same way, using the same interface, as downloaded videos. There's no special interface for it. It just pauses when its reached the end of the buffer (or current download point).
EDIT
Or do you mean the client explicitly seeks (forwards) to the end of the file? i.e. no normal "play" support?