Being in contact with a couple of chain stores and several independents—plus, of course, keeping an eye on eBay for any bargain deals—I'd say that here in the UK, Canon and Nikon's second hand markets are roughly equal; which bits of gear hold their value more or less depends on the specific model in question, with a clear divide between lenses and bodies.
For example, the Nikon 35mm sensor SLRs from ~10 years ago or so are holding their value much better than the Canon ones of the same era, especially the 5D2 which can now be bought cheaper than new entry-level APS-C SLRs. I would probably attribute this to the strong sales of the 5D2 in the first place, leading to the market then becoming flooded with them; more people bought them back then, so more people have one to sell now.
Conversely, the Canon 70-200 f/2.8 IS II costs nearly as much second hand as it does new (assuming it's in good condition, of course), while the Nikon equivalent of the same age sells for just about 60% of its original value. It's also much easier to find those Canon 70-200s in good condition, while the Nikons do tend to be a bit more worn down, even when described as being in good condition.
Cheaper stuff is more of a mixed bag. Looking at less-pro examples, the Canon 85mm f/1.8 holds its value very well, while the technically-superior-but-less-famous 100mm f/2 doesn't; for Nikon, both their mid-priced 85 and 105 lenses land right in the middle of the two Canons. None of them wear down or are any less or more tough than the others, and in practical, 'real world' terms, they're all optically up to scratch. In these cases it's a matter of raw popularity.
As a general rule, here in the UK, Nikon's pro bodies hold value better, while for lenses, it's Canon which retain value, and the same goes for condition; easy to find Canon L lenses in good condition, easy to find Nikon pro bodies in good condition. This shouldn't be at all surprising to anyone, given that the public perception for years now has been that Nikon have the superior bodies while Canon has the better lens range.
Though for what it's worth, go back 15 years and things were dead even, and go back 25 years and it was the total opposite. In another 5-10 years it'll probably be the opposite again. It's best to not worry too much about gear value in the long-term, and absolutely never think of gear as any kind of "investment". Buy what you need to get the photos you want/need. If the time comes to sell it out and you manage to get a fair chunk of the money back, think of that as a bonus. You could "invest" in the reportedly-toughest lenses and bodies on the planet, but they still won't retain their value if a front element gets smashed, a body falls out a 7th-story window, or it's all stolen. And you never know when the secondhand market will simply bottom-out, too. (That happened to Fuji cameras here very recently; bodies and lenses which had been holding their value very well suddenly dropped to bargain-basement prices and people have been really pissed off that they didn't sell their older Fujis sooner.)
Trust me, way prefer my SLR pictures, but damn it's annoying taking 20lbs of gear on a hike and more often now I'm ok with phone pics than say 3 yrs ago.
