Looking for a backup camera?

I'm getting into wedding photography, currently I have a Nikon d7000. I'm trying to decide between three different cameras.
Nikon d90 found one for 450
Nikon d300s found one for 700
Nikon d3200 new for around 600
This camera will be a backup/secondary camera. I will be using prime lenses on it most of the time, unless of course something happens to my d7000.
I'm looking for which one of these cameras would fit what I need?

I'm not a Nikon shooter, and I'm not interested in starting a brand debate, but I'll share my thoughts…
My main piece of advice would be to get a backup camera that uses the same batteries as your main one. That way you don't have to have two different chargers and two sets of batteries everywhere you go.
If you can't fit your current batteries into the body of those listed, see if it's possible to do it through a battery grip.

If you're based in a studio and have plenty of room, that may not be an issue, but if you are out shooting with limited space, a second charger and batteries can take up the same amount of room as a lens.

As for which body is the best, I'd say the one that most naturally matches the layout & controls of yor main one - again it's about making it easy for you. If a switch or control on one is in a greatly different position than on another, then it can take that extra few seconds to think before you click (a slight bugbear of mine when using my main bodies)

When it comes to the quality of the output from the camera (no, not just the sensor response, but the actual RAW/JPG file), if you're taking shots at ISO 100-400, shutter speeds faster than the focal length and middle range apertures (e.g. The good old f/8, 125) - i.e. Not pushing the camera anywhere near its limits, and you're not cropping the images down, or blowing them up to poster size, and you've got good lenses on the front without any filters on them, then there's very little noticeable difference in image quality in real life between a good shot taken with a D800 and a good shot taken with a good shot taken with a D3200. The people who tell you otherwise work in sales.

Good advice from Stewart re: batteries. I'll just add - scrub the D3200 from the list. It's an entry level model & has no focus motor in the body - any AF lenses you have will need manual focusing on it.

Personally I'd go for the D300s - a higher spec than the D90 & considered by Nikon to be part of their 'pro' line, so a Mg alloy body & weather sealed.