I think what you might want to do is try the closed source / SAAS route first either by locking in that 2nd customer or taking some investment. If it doesn't work, then start down the open source route.
It seems to me that all the knowledge / experience of your staff is your real asset here, not necessarily the code. While that, in theory, should mean open-sourcing the code won't matter for your business, in practice it means you will be seeding competitors unnecessarily.
Turning your staff into highly chargeable consultants could be a more sustainable business model than trying to herd the cats of the internet into trying to improve your product offering, when most of those guys won't have the experience of your existing team. By offering the code out as open source you are giving a bunch of people a leg up and cutting short your time as the only game in town, which puts extra pressure on sales and might not work out.