Criteria for choosing a BPMN tool (in no particular order):
1) Ease of use. You have to try a few to get a feel for this. Some tools are easy and fluid to use, and some aren't.
2) Support for the BPMN 2 standard syntax and semantics. Quite a few of the so-called "BPMN modelling" tools allow you to create diagrams that are illegal according to the BPMN 2 specification. This is because they are essentially just drawing tools with a BPMN stencil. Ensure that whatever tool you use supports all of the BPMN 2 specification accurately. "introduction to BPMN 2" is good guide for this is you don't want to dive into the BPMN 2 specification:
http://www.slideshare.net/jimarlow/introductiontobpmn005
3) Speed - is it quick to produce a diagram, or is it slow and fiddly?
4) Looks - do the BPMN 2 diagrams you can create look good?
5) Do you need UML integration as part of a wider modelling strategy? So perhaps a plug-in for a UML tool rather than a stand-alone BPMN tool.
6) Price!
7) What are your modelling needs now and in the foreseeable future?
8) Is the tool choice strategic (i.e. for corporate roll-out, you will wish to use it for the foreseeable future), or tactical (one off use).
9) Do you need multi-user support so more than one person can work on a model.
I'm sure other forum members can add to this list.