So a lot of what it comes down to is, how involved are you planning on getting in your investigations? If you are planning on going out to try and find and supply the ultimate proof, film might definitely be your avenue of choice. If you're just looking for a camera that takes decent pictures in the dark, and don't mind some criticisms about your findings, a decent digital camera should be fine.
I would agree with some of the previous posts on using what you already have. Give those cameras each a try, and see how you like them. Also, as previously posted, use a tripod. They work wonders when adjusting your camera to photograph better in the dark. Once you've gotten some practice using your cameras, then you'll know whether they work for what you need, and also if you're even interested enough to drop more money on equipment.
I will say this - I use a Canon Powershot. It's a six mega pixel, digital camera. I feel it takes good pictures in the dark, so long as you have a steady hand. These are usually flash pictures, so a tripod isn't usually necessary, but certainly does not hurt. My goal is primarily to document the surroundings in the area, though, not to "catch" something. Best of luck to you in whatever you decide to do - and keep us posted! We'd love to know more about what you catch, questions you have about your pictures, or even features of cameras you're looking into!