Can't comment on Nikon, but if you are looking at Canon, here is my experience.
In 2006 I bought a Canon 5D with 16-35L 2.8 USM, Ø77 Lens and a 28-135MM f3.5-5.6 IS Lens. I then took 7000 photographs in six weeks in Europe for a book I was writing. The 16-35 lens was and is the best lens I have used. It took sharp pictures at night with the only light being streetlights.
The 28-135 was such a disappointment, I returned it to the vendor and replaced it with the photojournalist's friend the 24-70mm 2.8 L USM, Ø77. The worst with the 28-135 was coming across a Greek wedding procession (complete with shotguns and musicians) setting myself up for closeups as they walked by a doorway for a rapid series of shots and finding when I got back to the office to process pics two months later, the auto focus simply could not keep up... fantastic opportunity lost. Also, when carrying it facing down, the lens would extend to the fullest extent due to gravity. The 24-70L extends, but stays where it belongs when pointing down.
The 5D is an amazing camera in every way except dust. All digital cameras get dust on the sensor, but the 5D shows more. I consider this to be a manufacturer's defect and have contemplated bringing a class-action suit to force them to call them back in. By coming out with the anti-dust shaker on the 400D they have made a defacto admission of the defect.
Then last year when I had driven 200 miles to do a photoshoot, the camera shorted out. Repaired under warranty where the technician told me that carrying lens on jets produces an ultrasonic vibration that can cause the stop screw to loosen, thus the lens can move to where it connects two contacts and the fuse shorts out. Problem is they buried the fuse deep in the camera, so kiss the 200 mile drive goodbye.
To prevent being in this position again, I purchased a 400D body as a backup. Uses the same lens, but the sensor is cropped so the lens act different (a 10-22 lens on a 400D acts like a 16-35 on a 5D).
The 400D was a huge surprise. It is a brilliant camera at one quarter the price of the 5D. Even though the sensor has fewer megapixels, the sensor is smaller, so the density is higher, meaning slightly better images. To understand what this means, see
http://www.cambridgeincolour.com/tutori ... r-size.htm
I had the 5D professionally cleaned by Canon when they fixed the fuse, so began photoshoots with both the 5D and the 400D at the same time, and a month later had the same number of photos with both (about 2,000 shots).
I then compared them for dust. The dust cleaner on the 400D kept it entirely clean... nothing. The 5D had visible dust on the images, requiring photoshop to remove them.
The 400D is far lighter, although with a heavy professional L lens on it, lightness only matters when taking carry on luggage. The 5D is limited to 8GB CF cards, the 400D takes the 16 and I hear may work with the 32GB. If you do a lot of photographs while travelling, the large card is good. Download the day's shots to a laptop or IPOD (I did both). Keep the IPOD in the hotel. Carry the laptop on your back, and camera in hand. Reduces the risk of losing photos in the event of theft or mugging.
The 400D's software is more advanced than its predecessor the 300D, and for the money, I can't imagine anything beating it. I do find that Picasa has a problem with the raw images of the 400D but likes the 5D raw just fine.
However, long term, the Canon repair guys explained that the 5D is a professional camera, meaning it can easily be repaired, parts are screwed together, whereas the 400D is all machine soldered. If something breaks after the warranty it's probably not worth fixing.
By the way, I live by the sea and it rains often. Does not seem to be a problem with either camera.
That's the camera body and the bottom line is that if you are just beginning, as it sounds from the tone of your questions, the 400D will be a fantastic camera for you.
But as someone said, its really about the lens. Good lenses cost big bucks, but they last, they hold their value and they make great pictures, and allow you to do a whole lot more with less than great raw material.
I own:
24 mm 2.8 - $275 at Amazon - older style but light, carry on 400D when size is an issue - due to cropped sensor it works like a 38 mm lens
50 mm 1.4 - $315 at Amazon - for the 5D when the light is really dim, like after dark on a Greek island. Also useful when you want to be discrete. Very small lens
16-35 2.8L - the best lens. $1,315 at Amazon - It is so sharp that I can take a huge wide shot, get back to the office, crop it drastically and get a sharp result. It's relatively light, not that long, and it does not extend externally when zoomed. On the 400D it is the equivalent of a 26-56 mm lens.
28-70 2.8L price about $1,071 at Amazon. The workhorse. Excellent quality, first rate, as would be expected. But it is bigger and heavier. The extra weight really does get tiring. Pointing at a candid shot makes you look like a spy, attracts attention. Extends externally. On a 400D it's like a 45-112, which is a bit zoomy... and on a 400D, it's all lens weight - off balance.
70-200 4.0 L zoom, about $550 without IS, $1,000 with IS (Image stablization - adjusts for hands shaking camera). I bought the IS model and find I mostly use it on the 400D where it is equivalent to a 112-300 mm lens. Curiously the extra weight and size works well here with the 400D, as it is nicely balanced in the middle of the lens, where the zoom is adjusted. Very sharp lens, perhaps sharper than the 16-35, but obviously a zoom. When you want candid shots of people, you can be far away enough they don't notice you. These newer 4.0L lens are smaller than the earlier 2.8L lenses, and you may want to look at the 17-35 4.0L to see what it is like. The 4.0L lenses are much cheaper than the 2.8L
If I had to take one lens on a trip, it would be the 16-35 F2.8L. If I then take a second, it is the much longer 70-200 F4.0L IS. For a while I carried the 400D and 24 mm lens in my pocket for the lightest camera when out and about, but gave up and got a Powershot 570 IS which actually fits in the pocket without tearing the jacket.
So what should you buy based on my experience? I would recommend the 400D, at about $500 to $600, and then buy the 16-35 2.8L professional lens. The package will set you back about $2,000, but will give you as good as you can get. If you have more money, wait for the full sensor 5D replacement with dust cleaning. The 5D is a real professional's camera, and there are a host of details where it delivers - subtle stuff. If they upgrade it with the stuff they put in the 400 and the 40D, it should be unbeatable.
I see there is a 450D coming out now, so check it out first. But before you take my advice, try out the EF17-40mm f/4L which is $688 at Amazon. I have no experience with it, but if it is as good as its 70-200 sibling, it should be great. Where it won't excel is in low-light. I took photos in Greek villages long after dark and used no flash with the 5D and 16-35. Oh, one other thing... the built in flash on the 400D is too low for the 16-35 and 28-70 lenses. There is a shadow on the bottom part of the image where the lense gets in the way of the flash. If you are doing flash shots, don't use a Ø77 lens or use an external, clip on flash.
Well that's my experience. Can't comment on Nikon. The last one I had was an 8008 and it fell apart. Still have my 1960 Nikormat, but haven't used it in 25 years.