Another D7000 unless you can get a D7100 cheaper.
This.
That said, why get a backup at all? cameras rarely fail and 'backups' often spend their lives in bags losing value. If you do NEED a backup the it is good to have the same camera so you can just pick it up and go, the D300 is a totally different camera, different layout, different features (and an older sensor).
I like to have 2 of the same bodies, as it is I have 3 but two is good. Minimises lens changes, but I use them both the same as one tends to have one lens on and the other the other
I used to have a D7000 and D7100. They were similar size and layout.....except for the plus and minus buttons that are in slightly different places but the opposite way around. I was always pressing the wrong one!!
I didn’t get much use of the D7000 after I got the D7100 because I found the AF was a little off on the D7000 after getting the D7100.