Ask HN: Web apps or desktop software? Where's the market?

This is a short question which might get some long answers. I want to develop a tool which makes it easy for teachers and professors to write tests: multiple version, multiple choice, long answer, scantron, the whole works. The question is, would I be better off doing this as a web service (subscription model) or a desktop application (buy-once model)? The attractive thing about this market is the target is academia, which often has a lot of money to spend on this sort of software. I'm just not sure which way to go with it.