I'm a newbie taper, but not a newbie when it comes to video. First of all, STAY AWAY FROM DVD CAMS! They record in lossy MPEG2 format. Why do that when you can record to MiniDV? It's kind if like taping in MP3 or WAV. Definately MiniDV, no questions, game over.
When it comes to taping long periods of time, LP mode is where it's at, but I'll raise you one too. Get yourself an 80 minute MiniDV tape, and THEN tape in LP mode, that'll give you 120 minutes on ONE TAPE! It's an easy trick to get the same quality footage over a continuous 2 hours instead of the typical one.
And YES, LP mode is not as robust as SP and it is not a defect of anyone's cams or the products. It has to do with the fact that when the tape is running that slow, it just barely has enough room to squeeze all of the data on in the amount of time it has, whereas SP mode has more tape to spread the same data over. Now, normally when played with the same cam within a short period of time, this will NEVER BE a problem. So, just capture the DV to your computer, and you get a perfect digital copy that is just as good as SP footage -- no difference. However, over time, the LP tapes may not play well or show mosaic blockiness, etc. This can happen because the cams heads change a little, and the LP mode is so unforgiving, that can become problematic. Also, pretty much every brand of MiniDV camera states that tapes recorded in LP mode on that brand may or may not work in other cams by the same manufacturer or in cams/decks of other manufacturers.
Bottom line IMHO: it's a tradeoff. You should probably stick with SP mode because of its reliable and widespread compatibility and robustness to last a while. But, when continuous time recording or your budget are the main concerns, then go with LP mode.