Why not do both? Release the application as open source and offer support and/or customizations for paying customers? Or release it under the GPL and require a commercial pay-for license from customere who want to integrate it into their own non-GPL applications (this is the scheme MySQL uses). This obviously depends on what exactly your application does, but there are quite a few business models that can be built around open-source software, maybe you just need to pick the one that fits best for you.

That being said, Moriarty is right, of course, do whatever you feel is best. Releasing your application as open source gives you a few advantages, like some free advertising and bug-testing, possibly even some contributions from other developers (though these may just be in the form of bug-reports). And that warm fuzzy feeling of course :-). But warm and fuzzy doesn't pay the bills and if you're looking to make money from this project a commercial license may be better for you.