Canon 5d mark 2 vs nikon d700?

Canon 5d mark 2 or nikon d700 hi iso 6400 low noise camera?

D700, albeit it might make sense to wait for its replacement, or the 5D3, each of which are due in the not so distant future.

The Pentax K5 also has good high ISO performance, and with image stabilization in the body you can have a stabilized fast prime lens, something which Nikon and Canon lack completely. That's pretty neat for handheld low light shooting if subject movement blur isn't an issue, thus enabling you to shoot at lower ISO values than with the other cameras.

Here they are, side by side:
http://www.dxomark.com/...d3)/Pentax

Here is a link were their sensors can be compared when used in low light

http://dxomark.com/...nd2)/Nikon

How to read those numbers:

http://dxomark.com/...sor-scores

Noise can be reduced both by using the noise reduction features in the cameras menu and when using Adobe Lightroom