I recently made the switch to Nikon and picked up a D800/D7100 combo. Having had the 7d also, I'll make some simple comparisons.
- AF Speed: Both cameras are super fast focusing and pretty accurate most of the time. Both are pretty customizable. The D7100 has 51 AF points and 24mp vs the 7d's 19 and 18. Canon's AF points are all cross type vs Nikon's 15.
- Build/Ergonomics: Two different camera bodies, both weather resistant, both felling completely different. The 7d has a rugged, hefty feel to it, while the 7100 feels slightly more compact and cheaper. Ergos are similar but different and just take some getting use too. Some are gonna like it, some not.
- Buffer: Nikon has gotten crucified for putting a small buffer in the 7100 - most testing is showing that the camera slows down greatly around a 9-shot burst and it takes several seconds to clear the buffer. If you never shoot longer bursts, no big deal. Decided advantage to the 7d if you shoot bursts.
- IQ: Both cameras are capable of excellent picture IQ. 7100 wins in the noise department though IMO. Easily cleaned up in PP and relatively fine and unnoticeable until ISO 3200.
- 1.3x Crop mode: Nikon includes this though I have made little to no use of it. You have the ability to "crop" in-camera and give your lens the appearance of using a 2x TC. Not sure on the math, but it's been discussed and proven, so a little research will help you here. It does cut the file from 24mp to about 15mp, so the only use I can see for doing it in-camera would be saving some card space.
Overall, I have been pleased with the 7100. I was also pleased with the 7d, less the noisy images. There's pros about both and cons about both. You really should go hold the 7100 and see if it fits you, literally and figuratively.
As for lenses, yes Nikon is a little behind on the mid-level platform. They have a 300 f4 which is highly regarded, but it lacks VR. No 400 5.6 and their version of the 100-400 (80-400 in Nikon world) smokes Canon's but costs double. 70-200 wise, a little focusing breathing but quality is on par with Canon.