Advantage: It handles all the parameter marshalling and invocation for you. To your service, it looks very much like a local service call (especially if you wrap the remote:invoke with a local service, which I’d recommend).
Disadvantage: Requires defining a remote server using Administrator (minor issue). Only works with another IS instance.
Using HTTP will remove the dependency on the remote service being hosted on IS. Calling via HTTP will be a little more involved, mapping inputs and outputs, catching errors and such. But overall, the level of effort is very different.
I’d offer that you create a wrapper service that calls the remote service. This will isolate it such that you can use either remote:invoke or HTTP and the service wrapper callers won’t care.