I suppose the easy answer is "it depends..." ;-)
Seriously, I've owned the D30 and the 1Ds, and for me, the D30 had a much shorter learning curve. Now, if you've used Canon's professional film cameras like the 1V, maybe the reverse would be true. You say you have some Canon glass - on which body(ies) have you used it?
If you're accustomed to making large prints with MF film, I think you'd be much happier with the 1Ds simply because of the extra resolution.
Also, since this is a one-time event, I think your results would be more predictably "good" with the 1Ds. Learning curves aside, the 1Ds has gotten very good reviews, while the 10D has given some people fits with focus issues.
I like your idea of shooting RAW + jpeg. I do the same thing, and it works well for me. I suggest setting the jpeg mode to small/fine so they won't take up too much space on top of the RAW files.
Speaking of space, if you're not shooting digitally now, have you estimated how much storage (int he form of CF cards) you'll need? Will you have access to a computer or other storage device so you can unload your CF cards? Or do you plan on purchasing or renting a huge amount of CF capacity for the weekend?
Last but not least, I highly recommend, whichever camera you choose, that you rent it two weekends in succession, if possible. The first weekend you can become familiar with the camera, and how it reacts with your lenses. You can experiment with the meter and the histogram in the kinds of light you expect to encounter when you photograph your neice. You can get a feel for how you'll transfer your images off your CF cards, and you might try processing a few in Photoshop to ensure you're comfortable with the white balance, etc. By doing this, when the blessed event happens, you'll be spending less time wondering about the camera, and more time making great images.
Hope This Helps!