My serious interest in photography began with my dissatifaction with my first dslr, an entry level Nikon (D3100). The problem wasn't that I didn't like the photos I could take with it but that I hated the ergonomics of the thing, and when the autofocus stopped working completely while I was on vacation in Paris I stopped trusting it and started investigating alternatives. For various reasons I ended up with a Pentax K-5, but after a while I decided I wanted to try FF and, as there's no such thing in Pentax-land, rented a 5DII (lingering anti-Nikon bias led me there rather than to a D700) and a couple of lenses and immediately jumped ship again - it was obvious that the picture quality was better, and the mechanical/focusing superiority of the Canon lenses, however well-built Pentax lenses can be, was shocking. Each time I jumped I sold all the old equipment; "lost" money on the bodies, as I had bought them new, but probably broke even overall on the lenses - those who worry about the cost of jumping ship probably shouldn't. To the extent I "lost" money I didn't - it was worth it for the use I had of the cameras and for the learning experience.
Since then I've wondered about Nikon FF and, out of sheer curiosity, rented D800e and D600 (+ a few lenses, of course) on the off-chance it seemed like an agreeable idea to add some Nikon equipment to my Canon gear, only to end up spending more on Canon. I disliked the ergonomics of their FF bodies as much as their intro bodies, and didn't think the lenses I tried were quite as good mechanically. I don't think there was any significant difference in the resulting images (leaving aside the superior resolution of the 800e and the better dynamic range of both, neither of which would be of much use to me most of the time), but I just don't much like taking photos with Nikon equipment: the mechanical act of taking photos just seems so much more pleasant with Canon (and Pentax). There are hardly any Nikon FF lenses for which I have lens envy, whereas I would have considerable lens envy if I switched to Nikon. So instead of adding Nikon, I've taken a rather different route and added smaller stuff - Olympus and Panasonic M43 (I prefer their ergonomics to Nikon's too...).
I can't answer your precise questions re APS-C equipment comparisons, but you might find it useful to try the lens comparisons at the digital picture, where you will be able to compare equivalent Canon and Nikon lenses, at least in terms of sharpness.