If you have $100 to spare, I'd recommend pre-ordering the photo cd: if you read the fine print of the offer, you'll find that as long as you don't actually use the credit in your account, you can cancel it for any reason whatsoever (e.g. don't like the pictures, didn't get enough pictures, don't want to spend the money any more). Once the first picture is taken on your photopass account, you're no longer able to get the offer.
I'd definitely have the photopass photographers take your picture both with their cameras and with yours (especially at night, sometimes their cameras are better equipped to take good pictures--if they have an external flash or a tripod, for example). Having two cameras take the shot also increases the chance that they'll get a good picture.
Then after the trip, review both your pictures and the photopass account and decide what you want. Are there unique poses on the photopass (either better than you got on your own camera or that you didn't get on your own camera) that you want to purchase? The math works like this: you'll have spent $99.95 for the cd. The cheapest print or digital download is $14.95. If there are fewer than 6 images on the photopass account that you want, cancel the prepaid cd and do individual downloads/prints (at most $89.70). If there are 7 or more images you want, it's a better deal to get the photopass cd.
I personally feel like the prices are high, but I value being able to get our entire group in a photo without having to bug some other guest to take our picture.