How does the source encoding reflect the settings used to export?
The quality you use to export the video for long term storage largely depends on what you will be doing with the footage at a later date.

If you will be mostly using it for personal viewing, a lower quality may be sufficient.  If you will be editing the footage and using it in another video, you will want to encode it a higher quality.

The only scenario I can think of for encoding to a higher quality than the original video is if you were including the clip for use with other videos of higher quality in a project that will ultimately be encoded with the higher quality.

To sum up:

You should encode the video in the highest quality you can without exceeding the original and without going over your space requirements.  Since large hard drives are relatively inexpensive these days, this will likely mean that you encode to the original quality of the video.