If you want a backup body, and you can afford it, I always suggest the SAME body as your main one. This way you have one system, and can switch seamlessly. I used to have 1Dx/1D4 combo, but the systems were different enough that I saved up, sold the 1D4, and got another 1Dx. It was well worth it because now I can just switch cameras instantly and start shooting.
For your question, I suggest another 5D3 if you need a 2nd body. If you don't need the second body really, then I wouldn't spend your hard-earned money.