I'm not familiar with the products on the market today, but I've done a little photography, and some of the terms that you are using are standard enough for me to give you some background. The bad news is that I'm going to write several paragraphs and end up telling you that I'm not entirely sure which camera will be best without knowing more. People who know me get used to this situation. Even so, you might find some of the information valuable.
A fellow engineer at work has done extensive research on the digital SLR market, and his conclusion is that Canon has the most advanced technology in their cameras. People getting the top of the line digital SLRs are usually getting Canons. Unfortunately, the Rebel really doesn't incorporate all of the technology. He started with a Rebel, upgraded to the next level, maybe D10, and then upgraded again to one that I think is called a D20. This camera is near professional grade. For your uses, either the Nikon or the Canon should be great, but if you find a hidden talent to make thousands of dollars as a photographer, you'll probably upgrade from either of these.
The f/ number on the Nikon lenses refers to the lowest aperture rating on the lens. When the camera is set at the lowest aperture number, for instance f3.5, the opening through which the light passes is as big as possible. While the camera is gathering all of this light, the optics on the lens need to be good, so the capacity for lower f numbers means that more quality went into the optics of that lens. All other things being equal, an f/1.8 lens is considerably better and will be considerably more expensive than an f/3.5 lens. For your purposes, these lenses should be good enough.
Understanding f-stops is important when shooting. When you're shooting in low light conditions, you want to be able to open the aperture as much as possible in order to gather light more quickly and use a higher shutter speed. The trade-off is that the bigger aperture means lower "depth of field." The depth of field is the distance over which things are in focus. For instance, if you focus on something 10 feet away at a small f-stop (large aperture), everything from 9 feet away to 11 feet away will be in focus. Everything else will be out of focus. If instead you take the same shot with a large f-stop (small aperture), everything from say 6 feet to 15 feet away might be in focus. Obviously, greater depth of field is nice for fixing minor focus mistakes, but sometimes keeping the background out of focus leads to more dramatic pictures.
The 18-55 mm and 55-200 mm refer to the focal length of the lens in millimeters. The range of focal lengths identify the lens as a zoom lens. Generally, a focal length of about 50 to 55 mm is considered to be 1X magnification. In other words, this focal length is supposed to show you a view that's about the same as what you get with the naked eye. Lower focal lengths give you wider-angle views. An 18 mm focal length is nearly a "fish-eye" lens, and I wouldn't be surprised if pictures taken with this lens show fish-eye distortions around the edges. A 200 mm focal length gets into the telephoto ranges and is similar to looking through a 4x binocular. If you want to get all of a big snake or a big cage in a shot but must remain close to the snake because you have a small room, the wide-angle focal lengths of the 18-55 lens will be helpful. If you want to be able to shoot a closer shot of something that you can't approach too closely, then the 55-200 mm lens will be helpful. With a zoom lens, you can choose how much you want the lens to magnify or reduce the image.
Another point about focal lengths is that apertures need to be smaller (f-stops larger) on lenses with longer focal lengths. (I don't remember the exact explanation. You're probably lucky that I don't.) When you see two f numbers given, for instance f/4-5.6 for the 55-200 zoom, the first number, f/4, is the minimum f-stop at the lower focal length, 55 mm. The second number, f/5.6, is the minimum f-stop at the higher focal length, 200mm.
The bad news after all of this explanation is that a feature that you really need is macro focusing, and your information doesn't tell whether any of these lenses offer macro focusing. Macro focusing roughly means being able to focus on things that are very close to the lens. Of course, "close" to a lens on the 55 mm focal length is not the same as "close" to a lens when set at the 200 mm focal length.
The 8.0 MP and 6.1 MP designations tell the maximum number of pixels in the image that each camera captures. More pixels means a sharper picture, but they also mean a bigger file. For what we post on these forums, the extra pixels in the Canon camera will not help you at all. You'll need to cut pictures from either camera by a huge amount to post them here. However, if you're going to take pictures and print them in 11x17 or some other big size, more pixels, or megapixels as they're called, will lead to sharper pictures.
I think both of these cameras will have all kinds of automatic focus and exposure options that most people enjoy using. Because I learned photography long ago with old 35 mm cameras, I tend to find these features annoying. Admitting that I find them annoying just makes me sound old, but there is a point to my ranting. I've had cameras with all kinds of automatic modes that just wouldn't take the shot that I wanted. I always recommend that people make sure that the camera has a mode that allows complete manual override. When the camera doesn't understand what you want, the override allows you to make the camera take the picture that you want. Admittedly, you still have to develop the photography skill to know what you want it to do, but when you have that skill, you don't want the camera pushing you around.
If the prices that people are quoting you are the same for these two set-ups, the Nikon is the better deal because you're getting two zoom lenses. One will give you wide-angle capability, and the other will give you telephoto capability. However, you need to be sure that one or both of these lenses have some macro focus capability. If not, you should look into upgrading the lens.
If you go with the Canon and get only one lens, I'd suggest trying to work a swap for a 35-105 or some similar size zoom lens. I don't think that you'd get much real value from the very low focal length part of the zoom, meaning the 18 mm part. You would get some value from a little bit of telephoto capability, meaning the 105 mm part of the lens I'm suggesting. I don't know what Canon is making, but lenses with designations like 35-70, 35-90, 35-135, 28-70, 28-105, etc. would all give similar performance. Again, none of these numbers guarantees macro focusing capability. You need to ask for that feature specifically.
No matter what you get, you'll need to practice.
Bill
PS. The scary thing about these subjects is that I'll be tempted to buy a digital SLR of my own. I don't need to spend that money right now.
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It's not how many snakes you have. It's how happy and healthy you can keep them.
