In my view, a backup camera needs to be as close to your main camera as possible, qualitywise. Or they should complement one another. A 5DII/1DIV is such a combo, a 5DII/7D a similar, still good, but cheaper alternative and a 5DIII/1DX a third in the higher end. If the quality differs too much, you'll never use the backup, unless your main is broken. I have the 5DIII/1DX combo and I use them simultanously, with different lenses.
So, for me, the alternaitve to getting a balanced combo like that, would rather be one of the higher quality compacts. Some really interesting alternatives are now available.