Old debate: Nikon vs Canon?

OK, I have saved for a while now, and am ready to switch from a 35mm film camera (kinda late, right?) to a dSLR. I'm an aspiring freelance photographer whose main interests include snapping nature/landscape & architectural photos. I have heard great things about both Canon and Nikon DSLR's. Here are the facts:
- I shoot a lot using HDR (High Dynamic Range), which I understand Nikon beats Canon on.
- I have heard from multiple trustworthy sources that Canon shoots better in nature/landscape scenes because their cameras have the technology to change ISO and shoot in low-light conditions better.
- I already have 2 (quite expensive) Nikon lenses, which I have shot with my friends Nikon, and have loved.So, it has to be a great argument against Nikon and for Canon for me to consider Canon.
- My budget is $1200, roughly.
- I have done some research, and a great starting camera is the Nikon D5000, with the Nikon Nikkor Macro lens which is what I also love (Macro Photography)
- I'm looking for the clearest image quality of the two.
The more opinions, the better. I really appreciate it!

I have Nikon and they are the best. But it actually depends on how you use it.day/night.

This question appears nearly every day on this forum. Tell you what, why don't you search through some of the older posts for opinions - they are many.
For me, Canon

"- I shoot a lot using HDR (High Dynamic Range), which I understand Nikon beats Canon on."
I'm not that big into HDR, but i'm not aware of any particular quality of Nikon cameras that make them superior to Canon for that purpose.
"- I have heard from multiple trustworthy sources that Canon shoots better in nature/landscape scenes because their cameras have the technology to change ISO and shoot in low-light conditions better. "
Again, this is a tie. Changing ISO is something that both do about equally well (albeit some Nikon cameras only start at 200 natve ISO while their direct Canon counterparts support a native 100 ISO ). On the other hand, in terms of low light/high ISO, Nikon held the edge in the past years. Canon closed up though, especially with the 550D, but early reviews of the D3100 indicate that Nikon is about to take another leap forward as well.
"- I already have 2 (quite expensive) Nikon lenses, which I have shot with my friends Nikon, and have loved.So, it has to be a great argument against Nikon and for Canon for me to consider Canon.
- My budget is $1200, roughly.
- I have done some research, and a great starting camera is the Nikon D5000, with the Nikon Nikkor Macro lens which is what I also love (Macro Photography)"
Already owning two expensive lenses pretty much should settle the issue. Keep in mind, coming from the 35mm film segment, they'll be compatible to full frame cameras. On the long run it may be a pity if you trade them for dedicated APS-C lenses on a Canon camera just to find out that in a few years you may upgrade to full frame…

I'm a Nikon guy, but it's really a toss up. Go to a store, handle the cameras you're interested in, see which is easier to navigate via menu, which to you feels better, etc. Then decide, you certainly don't have to buy the camera in that store.

1.bovine poo! Yes, nikon's sensor in the d5000/d90/d300 has a pretty awesome dynamic range but HDR can help any camera improve that.
2. Bovine poo again. Any halfway decent SLR will change the iso automatically if you set it to do that. Canon used to have the edge in low light work but with the current generation (minus the D3000) that's gone.
3. You didn not feel that you could trust us with the details on those so I can't really comment on that.chances are you can use the,
4.It's a nice camera with a good sensor. All depends on where you came from and how skilled you are. If you are a serious photographer I would also look into a lightly used D300.

Having two expensive Nikon lenses should settle your issue. You don't say what your lenses are, but if they aren't AF-S, then you should look at the D90 (or the replacement for the D90 which is coming out soon) which can handle AF lenses. The D5000 will only autofocus with AF-S lenses which have the focus motor in the lens.
The rest of your comments are a wash between the two manufacturers. Both brands give you roughly the same quality images.

I prefer Nikon D90 DX 12.3MP Digital SLR Camera which is good for you. It performs well.
* 12.3-megapixel DX-format CMOS imaging sensor
* 5.8x AF-S DX Nikkor 18-105mm f/3.5-5.6G ED VR lens included
* D-Movie Mode; Cinematic 24fps HD with sound
* 3-inch super-density 920, 000-dot color LCD monitor
* Capture images to SD/SDHC memory cards
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