It makes no difference at all. There isn't really any relationship between camcorder speed and DVD length.
That being said, it's usually better to record in SP mode because LP mode can cause more occasional dropouts and may not be as easy to read if you move the tape to a different camcorder.
I would submit that, philosophically, Chienworks is correct. From a practical matter, however, it may not make any difference. For example, I have used two cameras since 2000, a Sony TRV8 (my first), and a Sony VX2000 bought in 2003. Both cameras are used extensively, and I have gone through hundreds of miniDV tapes (TDK DVM60ME, bought at Costco in 6-packs for roughly $20 a pack). All tapes have been shot in the LP speed. I have never seen an interchangability issue between cameras and/or decks, and have only rarely seen dropouts. On my recent Lourdes shoot I brought home 6 tapes (9 hours) and saw two dropouts.
So my conclusion is that, as usually happens in all technologies, the standards have gotten so good over time that even what might have been considered iffy five or ten years ago is now very robust and stable. And I really enjoy the ability to shoot 90 minutes of a performance without having to change a tape.
I would suggest you experiment to see what your camera/tape combination will allow. I know old habits die hard, but I think you'll also find that LP carries no additional baggage for the added 30 minutes of shooting time.
I've had tapes come thru that were recorded in LP mode that could not be played back on our equipment without issues. A horizontal banding was the most common issue, sometimes bad, sometimes not as bad.
It appears to be an alignment issue with the camera that recorded the tapes - invariably the tapes with issues were recorded later in the life of the camera since the tapes taken early on were fine.
"For example, I have used two cameras since 2000, a Sony TRV8 (my first), and a Sony VX2000 bought in 2003."
In your case you were using two cameras from the same manufacturer.
I have worked with about two dozen tapes shot on the LP setting in a Sony TRV 11. I eventually had to give up trying to get useful results in capturing them using a Canon camcorder. Truly awful audio results.
The same tapes produced fine results when captured using the original Sony TRV 11camcorder.
I'm not experiencing as good luck as riredale with LP.
It seems that the camera that shot the tapes is the best as reading the tape back- other cameras or devices may have dropout issues with the tape.
I have a Sumsung camcorder (terrible camera) and it has major problems reading LP tapes from another camera. It's broken now (again, terrible camera).