Having done the Nikon to Canon switch 3 months ago from a D90 to a 5D2, I have my regrets, but have come to like Canon for what it is (and is not).
Pros:
- Canon beats the shits out of Nikon in terms of awesome AWESOME prime lenses. For example, you want wide? There are a myriad of excellent fast primes. Want telephoto? The same.
- Canon lenses are much cheaper, both brand new and secondhand. Canon bodies are overpriced but the cheaper lenses make up for the difference thrice fold.
- L-lens GAS (could be a con, not too sure on this yet)
Cons:
- The AF system on the 5D series is atrocious when comparing to Nikon's offering on even their entry-level DSLRs. You will curse and swear at the fact that the spread of AF points is useless and that all but the centre AF point works best. For example, on my lowly D40, I could choose any of the three AF points and use them perfectly. Don't bother with the 5D series. Just use the centre Af and recomposs.
- Build quality of the 5D feels extremely cheap when compared to the price you have paid and compared to the Nikon equivalent (D700).
- Lighting system. ETTL works but in my experience is years behind Nikon's ITTL both in terms of user-friendliness and function. Slap on a S400/600/800/900 on any Nikon DSLR and it will work as is in all program modes. Slap on a 430EX/580EX on any Canon DSLR and you have to fiddle with custom functions to make sure the blasted camera knows that there is a stupid external attached to its equally stupid hotshoe.
- Stupid out-dated menus such as the burried settings within Custom Functions. Have fun decoding all that!
IQ is the same. Noise is the same to my eyes.
Stick to Nikon - not because you have invested in it, but purely because you already are familiar with the Nikon system. A head-first dive into Canon's system will only cause you frustration.
With that said, it took be a good 3 months to appreciate the 5D, 5Dii, 1DmIII and 1DsmII for what they were. I had to forget my Nikon ways and once I did, loved my Canon, in particular, the lenses (*hugs 24L, 50 1.4, 70-200L, 200L, 17-40L, 100L*).
The grass is always greener on the other side. The same applies once you've crossed over.