Now the question: is it a good idea to put the platform open-source?
It will greatly depend on your business strategy, Open Source can be a friend or a project killer, like any other license option. GPL will probably be a bad option, not that GPL is a bad license but it tends to drive programmers and companies away from the use of the platform for fear of the "GPL contagious nature", but there are other licenses one can use, LGPL, Apache, a license use heavily by Google, etc.
Open Source can work and provide you income. You'll have an open platform, one programmers can look at, understand how it works, know if a given bug is in their code or in yours. You'll have a community, even if small, that will support you and help the project grow.
On the down side, you'll have code that you competitors will be able to see, but understand that to use it, their product will probably need to be Open Source also, this will depend on the chosen license. You'll also be unable to stop anyone from selling your product and those sales you'll give you no return.
If you go for a closed license you'll have the trust of some companies that don't understand and are actually afraid of Open Source, it is still a business model that some managers can't comprehend. And every license you sell you be for money, not that that money will pay for the costs, but you have a tighter control over you software. That control drives many other away.
You can go for the dual license option, but that has a few problems, code that is contributed to the Open Source version cannot be included in the close version without the author's agreement, and that can cause you problems. It will be hard to take advantage of the contributions made by any community created.
My view is probably biased as I'm an Open Source supporter, I work with some groups and contribute to some projects with my free time and meager skills
, but I would go for Open Source.
If you choose Open Source, please don't change the license scheme to proprietary afterwards, it will drive your customers away, it is really seen as a trust breach. I can give you the example o activeCollab, the creator decided that version 1.0 was going to be closed, half his clients moved to other projects, some of them even created forks based on the previous opened code.
1. I can't sell it anymore. Well, I can, technically create separate licenses for a commercial and not-commercial people. But I guess that less people will pay for what can be taken for free. At least outside US.
Just to point out that nothing in Open Source license prevents you from selling the software. You can sell it, and many companies actually by Open Source software. The ones that want the source are always programmers that, at some point, want to look at the code. Generally, people don't need the code, and I can barely remember when I accessed the source of any Open Source software I downloaded.
So, what to make of all of this? It really is up to you, you're the one that's going to have to choose, depending on your target audience, your country, the legislation you have to abide to, etc.
Personally I would go for one of the various approved OSI license and preferably one that is GPL compatible, but that's me