NixOps is a tool for "spinning up" nixos machines. Think: ec2 API.
Disnix is a toolset for managing the services running on a network of machines. Think: Chef/Puppet for multiple hosts.
Using DisnixOS (see the latest builds) the two can work together well, delegating to NixOps for instantiation of servers and delegating to Disnix to configure all your services on those servers.
If you are just deploying to a single server or the relationships between your services are simple, then NixOps can probably do everything you need. If you have multiple machines, running multiple services that are all subtly inter-related... then you will want Disnix/DisnixOS.