DVD Studio Pro 3 subtitle stream

[BAF]

I've got to include a subtitle stream on a dvd for a client and despite some research I'm still not sure how to approach this. Would the stream be:

- a subtitle track that gets superimposed over the video asset once the subtitle option is engaged. Done in DVD studio pro.

-or, a totally separate video asset with subtitles already on them that plays in place of the other asset when option is engaged. Subtitles layed in final cut pro. I did an audio commentary that worked like this. It gave the illusion of the commentary being over the normal audio but was actually an entirely different audio track.

If I know where to start I can figure out the rest.
Thanks

[Case]

The first option would be the right choice. The second is technically possible, but much more complicated and with the downside of having half the bitrate available per video stream.