I work for Atlassian. It's a $100M business. It wouldn't fall into either category. We've gotten there by selling a lot of software over the internet. We spend hardly anything on customer acquisition. There is no viral effect to our products.
Another successful business I can think of off the top of my head is ZenDesk. They have to be approaching $100M. Their main sales channel is the internet (low customer acquisition). They have zero viral effect. You either have X support staff or you don't therefore you will need an X-user ZenDesk license.
You certainly know your market better than I do, but it seems to me that you are taking advantage of a viral effect. As people use Jira (for instance) and they move around from one company to another, they're likely to recommend using Jira at their new places when they get sick of Rational, Remedy, Bugzilla, etc.
People using Jira and realizing that there is a bug tracker that doesn't stink are very likely to bring that message to their next company. The word about a great product spreads... like a virus, albeit a slower moving one than pet rocks.