Open-source or proprietary?

I am part of a team developing a new software technology, and we have a bit of a dilemma: whether to make it open source or proprietary? The product in question will be a core technology -- one could call it a platform -- on top of which specific applications will be built, either by us or by other developments.

We are aware of the ups and downs of either approach, but are not certain what impact will they have. On one hand, open-sourcing it would make us able to create a wider base of developers familiar with it; on the other hand, we plan to build a profitable business on top of it, and fear that investors (whom we need to fund the R&D) will be driven away by the incomplete ownership of the core.

What is your experience here? Companies such as OpenX and SugarCRM seem to be doing quite fine, but then they might be the extremes -- we'd never really hear of the failures, would we?