Hi,
According to the Advanced routing guide:
"Each instance contains a fully independent OSPF database, and routes from one domain are not automatically advertised to another domain"
I think this means that 1 devices routing table will have routes from both OSPF instances, and routes that clash will be ranked according to routing rules.
VRF in the other hand creates 2 distinct routing tables, and this is different from ospf instances.
This is how i understood it, maybe im wrong, feel free to correct me.