Nikon vs. Canon:
I think Nikon has the better lens selection; while all of Canon's equivalent lenses tend to be slightly less expensive. The Nikon D7000 is the only Nikon body with both full-manual exposure control over ISO, shutter speed, and aperture in D-Movie mode, and which records in the AVCHD format, which employs a modern, efficient, MPEG-4 based compression algorithm (also referred to as a, "CODEC," which stands for "compression/decompression").
If you need both 1080p24 video and stills in a full-frame body NOW, then the Canon 5D is your only choice.
If only considering Nikon, I would only recommend the D7000 for video right now. The D90's video is very substandard, and suffers from three major video artifacts: "stairstepping," color aliasing (color banding), and 60Hz roll bars. Additionally, the D90 offers zero control over ISO and shutter speed in D-Movie mode, making producing professional video results difficult at best. While the D3s' video is very good, it uses an older compression algorithm, which I wouldn't recommend.
Nikon video-capable bodies:
NIkon D90/D5000: DX-format; 720p24; motion-JPG CODEC; no manual control in D-Movie mode; video artifacting.
Nikon D300s: DX-format; 720p24; motion-JPEG CODEC; (manual control unknown).
Nikon D7000: DX-format; 1080p24; AVCHD CODEC; manual control in D-Movie mode.
Nikon D3100: DX-format; 1080p24; AVCHD CODEC; no manual control in D-Movie mode.
Nikon D3s: FX-format; 720p24; motion-JPEG CODEC; manual control in D-Movie mode.
Don't buy slow zooms (lenses with high, maximum numerical apertures, e.g., f/3.5, f/5.6). Their large light-quantity requirements make them impractical for many professional applications. Buy either fast zooms (f/2.8), or fast fixed lenses (f/1.4-f/2.8).