On first impressions, since I have no experience with OpenGrok nor Sourcegraph, they look to serve about the same core need and provide most the same core functionality, but Sourcegraph is about what I would expect from a company providing usability and features on top of what is generally available for free.
That is, Sourcegraph looks to compare to OpenGrok like Github compares to Gitweb. At least from a cursory look.
Considering that Sourcegraph doesn't support C or C++ and those are pretty much the only languages I write that have less than perfect IDE crossrefence support I don't really see a point in Sourcegraph at the moment.
Didn't one of the sourcegraph founders post a link to OpenBSD in sourcegraph at the top of this thread? That's C. Am I misunderstanding what you're taking about?
Someone has it run on the openbsd code here: http://bxr.su/OpenBSD/ and it should produce a much more useful representation of the code, see http://bxr.su/OpenBSD/lib/libutil/bcrypt_pbkdf.c#98
Mozilla also has one called DXR which is designed for their large, C++ heavy codebases: https://wiki.mozilla.org/DXR