Hi,
You just have to balance price against features.
If you are looking at entry-level cameras, the Nikon D40 and D40x have limited lens selection. Unlike more expensive models, they can only use the lenses with an auto focus motor built in, which excludes roughly half of the current AF Nikkors and all the older manual focus lenses. I believe the D80 and up can use the widest selection of selection lenses. The reason for this is that Nikon has made a real effort to retain backward compatibility to their AI/AI-S manual focus lenses, dating back as far ad the 1960s or 70s. But now, in order to get to the lower, entry level price point, they find themselves leaving the AF motor out of the camera bodies on those models, which so many of their current and recent lenses still require.
Meanwhile Canon entry-level models all are compatible with all Canon autofocus lenses made since the 1990s and on. No modern cameras work very well with Canon's old FD mount manual focus lenses. Canon made clean break (painful at the time) from their old system and has worked hard on 100% compatibility ever since.
Some Nikon models still use CCD sensors (entry-level models). Personally, I prefer CMOS, which Canon devoted itself to early on and has progressed a great deal with. To me, CMOS gives a cleaner image (less noise). Apparently Nikon agrees. All their newer, higher end models now employ CMOS. (Sony and Olympus are the other two manuf. who use mostly or entirely CMOS now.)
Tied to the CCD vs CMOS sensor issue, Nikon owners I work alongside have been reluctant to set their cameras above ISO 400, maybe 800 in a pinch. I don't think twice about using ISO 1600 on my two 30Ds and 10D, and have used ISO 3200 pretty successfully in many situations (Canon 5D/1D Mk III owners don't think twice about using ISO 3200).
Many felt the Nikon CCD images got "plasticky" above certain ISOs, and lost quite a bit of detail. That's because they needed to use very aggressive noise reduction. Canon didn't need to use as aggressive noise reduction with their CMOS. In fact, it seems Nikon has continued to use pretty aggressive noise reduction, even now with their new models with CMOS chips, still leading to some loss of detail although noise has been greatly reduced. Perhaps NR can be turned of or is adjustable, though. I haven't personally shot with the D300 or D3, so this is really just an observation of some comments I've read and heard. There's a lot of cheering in the Nikon camp, though, to be able to set ISO 800 and above without too much worry now.
There are quite a few more Canon prime lenses to choose from, and a more limited selection of prime Nikkors (primes are non-zoom lenses) in the current lens line ups. I use zooms when I must, but really prefer to work with primes whenever they are practical. Often in Canon's lens line-up you will see two or three choices of primes, with other manufacturers including Nikon it's seldom more than one version in each focal length. Not every one prefers prime lenses the way I do, so this may or may not be important.
When I first got into the Canon system, they really were the only game in town in terms of Image Stabilization. That was an important feature for me. Today Nikon has a lot of lenses with their own VR system, although still not quite as many as Canon's IS. Other manufacturers are also catching up with their own methods of image stabilization.
Canon isn't perfect. No system is. Nikon has recently released new 14-24/2.8 lens that looks to be terrific. They have also had a 200/2 VR lens in their line-up for a while. Canon is finally getting around to producing a comparable lens again soon.
So definitely look carefully at all your options. If you wife has a number of older, manual focus or even earlier AF lenses, one of the more advanced Nikon models that can use them might be just the ticket.
Olympus, Pentax, Sony and others are putting out some excellent offerings, too.
Nice Anniversary gift idea, by the way. Best of luck with your shopping!
